13 min read

Visualizing ggplot2 internals with shiny and D3

TL;DR – I built this shiny app to visualize ggplot2 internals.

I’m fortunate enough to be participating in Google’s Summer of Code program where I’m helping develop the R package animint. This package is one of many attempts to bring interactive web graphics to the R console. Animint’s approach is somewhat unique in it’s translation of ggplot2 code to HTML/SVG output. To do this, animint first compiles a list of ggplot objects and extracts the parts necessary for rendering output. Although the language is incredibly expressive and powerful for users, as any ggplot2 developer could tell you, the structure underlying a ggplot object is quite complicated (sometimes, even the original author needs help).

library(ggplot2)
p <- ggplot(mtcars, aes(mpg, wt)) +
 geom_point(colour='grey50', size = 4) +
 geom_point(aes(colour = cyl)) + facet_wrap(~am, nrow = 2)
str(p)
## List of 9
##  $ data       :'data.frame': 32 obs. of  11 variables:
##   ..$ mpg : num [1:32] 21 21 22.8 21.4 18.7 18.1 14.3 24.4 22.8 19.2 ...
##   ..$ cyl : num [1:32] 6 6 4 6 8 6 8 4 4 6 ...
##   ..$ disp: num [1:32] 160 160 108 258 360 ...
##   ..$ hp  : num [1:32] 110 110 93 110 175 105 245 62 95 123 ...
##   ..$ drat: num [1:32] 3.9 3.9 3.85 3.08 3.15 2.76 3.21 3.69 3.92 3.92 ...
##   ..$ wt  : num [1:32] 2.62 2.88 2.32 3.21 3.44 ...
##   ..$ qsec: num [1:32] 16.5 17 18.6 19.4 17 ...
##   ..$ vs  : num [1:32] 0 0 1 1 0 1 0 1 1 1 ...
##   ..$ am  : num [1:32] 1 1 1 0 0 0 0 0 0 0 ...
##   ..$ gear: num [1:32] 4 4 4 3 3 3 3 4 4 4 ...
##   ..$ carb: num [1:32] 4 4 1 1 2 1 4 2 2 4 ...
##  $ layers     :List of 2
##   ..$ :Classes 'LayerInstance', 'Layer', 'ggproto', 'gg' <ggproto object: Class LayerInstance, Layer, gg>
##     aes_params: list
##     compute_aesthetics: function
##     compute_geom_1: function
##     compute_geom_2: function
##     compute_position: function
##     compute_statistic: function
##     data: waiver
##     draw_geom: function
##     finish_statistics: function
##     geom: <ggproto object: Class GeomPoint, Geom, gg>
##         aesthetics: function
##         default_aes: uneval
##         draw_group: function
##         draw_key: function
##         draw_layer: function
##         draw_panel: function
##         extra_params: na.rm
##         handle_na: function
##         non_missing_aes: size shape colour
##         optional_aes: 
##         parameters: function
##         required_aes: x y
##         setup_data: function
##         use_defaults: function
##         super:  <ggproto object: Class Geom, gg>
##     geom_params: list
##     inherit.aes: TRUE
##     layer_data: function
##     map_statistic: function
##     mapping: NULL
##     position: <ggproto object: Class PositionIdentity, Position, gg>
##         compute_layer: function
##         compute_panel: function
##         required_aes: 
##         setup_data: function
##         setup_params: function
##         super:  <ggproto object: Class Position, gg>
##     print: function
##     show.legend: NA
##     stat: <ggproto object: Class StatIdentity, Stat, gg>
##         aesthetics: function
##         compute_group: function
##         compute_layer: function
##         compute_panel: function
##         default_aes: uneval
##         extra_params: na.rm
##         finish_layer: function
##         non_missing_aes: 
##         parameters: function
##         required_aes: 
##         retransform: TRUE
##         setup_data: function
##         setup_params: function
##         super:  <ggproto object: Class Stat, gg>
##     stat_params: list
##     subset: NULL
##     super:  <ggproto object: Class Layer, gg> 
##   ..$ :Classes 'LayerInstance', 'Layer', 'ggproto', 'gg' <ggproto object: Class LayerInstance, Layer, gg>
##     aes_params: list
##     compute_aesthetics: function
##     compute_geom_1: function
##     compute_geom_2: function
##     compute_position: function
##     compute_statistic: function
##     data: waiver
##     draw_geom: function
##     finish_statistics: function
##     geom: <ggproto object: Class GeomPoint, Geom, gg>
##         aesthetics: function
##         default_aes: uneval
##         draw_group: function
##         draw_key: function
##         draw_layer: function
##         draw_panel: function
##         extra_params: na.rm
##         handle_na: function
##         non_missing_aes: size shape colour
##         optional_aes: 
##         parameters: function
##         required_aes: x y
##         setup_data: function
##         use_defaults: function
##         super:  <ggproto object: Class Geom, gg>
##     geom_params: list
##     inherit.aes: TRUE
##     layer_data: function
##     map_statistic: function
##     mapping: uneval
##     position: <ggproto object: Class PositionIdentity, Position, gg>
##         compute_layer: function
##         compute_panel: function
##         required_aes: 
##         setup_data: function
##         setup_params: function
##         super:  <ggproto object: Class Position, gg>
##     print: function
##     show.legend: NA
##     stat: <ggproto object: Class StatIdentity, Stat, gg>
##         aesthetics: function
##         compute_group: function
##         compute_layer: function
##         compute_panel: function
##         default_aes: uneval
##         extra_params: na.rm
##         finish_layer: function
##         non_missing_aes: 
##         parameters: function
##         required_aes: 
##         retransform: TRUE
##         setup_data: function
##         setup_params: function
##         super:  <ggproto object: Class Stat, gg>
##     stat_params: list
##     subset: NULL
##     super:  <ggproto object: Class Layer, gg> 
##  $ scales     :Classes 'ScalesList', 'ggproto', 'gg' <ggproto object: Class ScalesList, gg>
##     add: function
##     clone: function
##     find: function
##     get_scales: function
##     has_scale: function
##     input: function
##     n: function
##     non_position_scales: function
##     scales: list
##     super:  <ggproto object: Class ScalesList, gg> 
##  $ mapping    :List of 2
##   ..$ x: symbol mpg
##   ..$ y: symbol wt
##  $ theme      : list()
##  $ coordinates:Classes 'CoordCartesian', 'Coord', 'ggproto', 'gg' <ggproto object: Class CoordCartesian, Coord, gg>
##     aspect: function
##     distance: function
##     expand: TRUE
##     is_linear: function
##     labels: function
##     limits: list
##     modify_scales: function
##     range: function
##     render_axis_h: function
##     render_axis_v: function
##     render_bg: function
##     render_fg: function
##     setup_data: function
##     setup_layout: function
##     setup_panel_params: function
##     setup_params: function
##     transform: function
##     super:  <ggproto object: Class CoordCartesian, Coord, gg> 
##  $ facet      :Classes 'FacetWrap', 'Facet', 'ggproto', 'gg' <ggproto object: Class FacetWrap, Facet, gg>
##     compute_layout: function
##     draw_back: function
##     draw_front: function
##     draw_labels: function
##     draw_panels: function
##     finish_data: function
##     init_scales: function
##     map_data: function
##     params: list
##     setup_data: function
##     setup_params: function
##     shrink: TRUE
##     train_scales: function
##     vars: function
##     super:  <ggproto object: Class FacetWrap, Facet, gg> 
##  $ plot_env   :<environment: R_GlobalEnv> 
##  $ labels     :List of 3
##   ..$ x     : chr "mpg"
##   ..$ y     : chr "wt"
##   ..$ colour: chr "cyl"
##  - attr(*, "class")= chr [1:2] "gg" "ggplot"

Of course, this structure gets much more complicated after we “build” the plot –

str(ggplot_build(p))
## List of 3
##  $ data  :List of 2
##   ..$ :'data.frame': 32 obs. of  10 variables:
##   .. ..$ x     : num [1:32] 21.4 18.7 18.1 14.3 24.4 22.8 19.2 17.8 16.4 17.3 ...
##   .. ..$ y     : num [1:32] 3.21 3.44 3.46 3.57 3.19 ...
##   .. ..$ PANEL : Factor w/ 2 levels "1","2": 1 1 1 1 1 1 1 1 1 1 ...
##   .. ..$ group : int [1:32] -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 ...
##   .. ..$ shape : num [1:32] 19 19 19 19 19 19 19 19 19 19 ...
##   .. ..$ colour: chr [1:32] "grey50" "grey50" "grey50" "grey50" ...
##   .. ..$ size  : num [1:32] 4 4 4 4 4 4 4 4 4 4 ...
##   .. ..$ fill  : logi [1:32] NA NA NA NA NA NA ...
##   .. ..$ alpha : logi [1:32] NA NA NA NA NA NA ...
##   .. ..$ stroke: num [1:32] 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 ...
##   ..$ :'data.frame': 32 obs. of  10 variables:
##   .. ..$ colour: chr [1:32] "#336A98" "#56B1F7" "#336A98" "#56B1F7" ...
##   .. ..$ x     : num [1:32] 21.4 18.7 18.1 14.3 24.4 22.8 19.2 17.8 16.4 17.3 ...
##   .. ..$ y     : num [1:32] 3.21 3.44 3.46 3.57 3.19 ...
##   .. ..$ PANEL : Factor w/ 2 levels "1","2": 1 1 1 1 1 1 1 1 1 1 ...
##   .. ..$ group : int [1:32] -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 ...
##   .. ..$ shape : num [1:32] 19 19 19 19 19 19 19 19 19 19 ...
##   .. ..$ size  : num [1:32] 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 ...
##   .. ..$ fill  : logi [1:32] NA NA NA NA NA NA ...
##   .. ..$ alpha : logi [1:32] NA NA NA NA NA NA ...
##   .. ..$ stroke: num [1:32] 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 ...
##  $ layout:Classes 'Layout', 'ggproto', 'gg' <ggproto object: Class Layout, gg>
##     coord: <ggproto object: Class CoordCartesian, Coord, gg>
##         aspect: function
##         distance: function
##         expand: TRUE
##         is_linear: function
##         labels: function
##         limits: list
##         modify_scales: function
##         range: function
##         render_axis_h: function
##         render_axis_v: function
##         render_bg: function
##         render_fg: function
##         setup_data: function
##         setup_layout: function
##         setup_panel_params: function
##         setup_params: function
##         transform: function
##         super:  <ggproto object: Class CoordCartesian, Coord, gg>
##     coord_params: list
##     facet: <ggproto object: Class FacetWrap, Facet, gg>
##         compute_layout: function
##         draw_back: function
##         draw_front: function
##         draw_labels: function
##         draw_panels: function
##         finish_data: function
##         init_scales: function
##         map_data: function
##         params: list
##         setup_data: function
##         setup_params: function
##         shrink: TRUE
##         train_scales: function
##         vars: function
##         super:  <ggproto object: Class FacetWrap, Facet, gg>
##     facet_params: list
##     finish_data: function
##     get_scales: function
##     layout: data.frame
##     map_position: function
##     panel_params: list
##     panel_scales_x: list
##     panel_scales_y: list
##     render: function
##     render_labels: function
##     reset_scales: function
##     setup: function
##     setup_panel_params: function
##     train_position: function
##     xlabel: function
##     ylabel: function
##     super:  <ggproto object: Class Layout, gg> 
##  $ plot  :List of 9
##   ..$ data       :'data.frame':  32 obs. of  11 variables:
##   .. ..$ mpg : num [1:32] 21 21 22.8 21.4 18.7 18.1 14.3 24.4 22.8 19.2 ...
##   .. ..$ cyl : num [1:32] 6 6 4 6 8 6 8 4 4 6 ...
##   .. ..$ disp: num [1:32] 160 160 108 258 360 ...
##   .. ..$ hp  : num [1:32] 110 110 93 110 175 105 245 62 95 123 ...
##   .. ..$ drat: num [1:32] 3.9 3.9 3.85 3.08 3.15 2.76 3.21 3.69 3.92 3.92 ...
##   .. ..$ wt  : num [1:32] 2.62 2.88 2.32 3.21 3.44 ...
##   .. ..$ qsec: num [1:32] 16.5 17 18.6 19.4 17 ...
##   .. ..$ vs  : num [1:32] 0 0 1 1 0 1 0 1 1 1 ...
##   .. ..$ am  : num [1:32] 1 1 1 0 0 0 0 0 0 0 ...
##   .. ..$ gear: num [1:32] 4 4 4 3 3 3 3 4 4 4 ...
##   .. ..$ carb: num [1:32] 4 4 1 1 2 1 4 2 2 4 ...
##   ..$ layers     :List of 2
##   .. ..$ :Classes 'LayerInstance', 'Layer', 'ggproto', 'gg' <ggproto object: Class LayerInstance, Layer, gg>
##     aes_params: list
##     compute_aesthetics: function
##     compute_geom_1: function
##     compute_geom_2: function
##     compute_position: function
##     compute_statistic: function
##     data: waiver
##     draw_geom: function
##     finish_statistics: function
##     geom: <ggproto object: Class GeomPoint, Geom, gg>
##         aesthetics: function
##         default_aes: uneval
##         draw_group: function
##         draw_key: function
##         draw_layer: function
##         draw_panel: function
##         extra_params: na.rm
##         handle_na: function
##         non_missing_aes: size shape colour
##         optional_aes: 
##         parameters: function
##         required_aes: x y
##         setup_data: function
##         use_defaults: function
##         super:  <ggproto object: Class Geom, gg>
##     geom_params: list
##     inherit.aes: TRUE
##     layer_data: function
##     map_statistic: function
##     mapping: NULL
##     position: <ggproto object: Class PositionIdentity, Position, gg>
##         compute_layer: function
##         compute_panel: function
##         required_aes: 
##         setup_data: function
##         setup_params: function
##         super:  <ggproto object: Class Position, gg>
##     print: function
##     show.legend: NA
##     stat: <ggproto object: Class StatIdentity, Stat, gg>
##         aesthetics: function
##         compute_group: function
##         compute_layer: function
##         compute_panel: function
##         default_aes: uneval
##         extra_params: na.rm
##         finish_layer: function
##         non_missing_aes: 
##         parameters: function
##         required_aes: 
##         retransform: TRUE
##         setup_data: function
##         setup_params: function
##         super:  <ggproto object: Class Stat, gg>
##     stat_params: list
##     subset: NULL
##     super:  <ggproto object: Class Layer, gg> 
##   .. ..$ :Classes 'LayerInstance', 'Layer', 'ggproto', 'gg' <ggproto object: Class LayerInstance, Layer, gg>
##     aes_params: list
##     compute_aesthetics: function
##     compute_geom_1: function
##     compute_geom_2: function
##     compute_position: function
##     compute_statistic: function
##     data: waiver
##     draw_geom: function
##     finish_statistics: function
##     geom: <ggproto object: Class GeomPoint, Geom, gg>
##         aesthetics: function
##         default_aes: uneval
##         draw_group: function
##         draw_key: function
##         draw_layer: function
##         draw_panel: function
##         extra_params: na.rm
##         handle_na: function
##         non_missing_aes: size shape colour
##         optional_aes: 
##         parameters: function
##         required_aes: x y
##         setup_data: function
##         use_defaults: function
##         super:  <ggproto object: Class Geom, gg>
##     geom_params: list
##     inherit.aes: TRUE
##     layer_data: function
##     map_statistic: function
##     mapping: uneval
##     position: <ggproto object: Class PositionIdentity, Position, gg>
##         compute_layer: function
##         compute_panel: function
##         required_aes: 
##         setup_data: function
##         setup_params: function
##         super:  <ggproto object: Class Position, gg>
##     print: function
##     show.legend: NA
##     stat: <ggproto object: Class StatIdentity, Stat, gg>
##         aesthetics: function
##         compute_group: function
##         compute_layer: function
##         compute_panel: function
##         default_aes: uneval
##         extra_params: na.rm
##         finish_layer: function
##         non_missing_aes: 
##         parameters: function
##         required_aes: 
##         retransform: TRUE
##         setup_data: function
##         setup_params: function
##         super:  <ggproto object: Class Stat, gg>
##     stat_params: list
##     subset: NULL
##     super:  <ggproto object: Class Layer, gg> 
##   ..$ scales     :Classes 'ScalesList', 'ggproto', 'gg' <ggproto object: Class ScalesList, gg>
##     add: function
##     clone: function
##     find: function
##     get_scales: function
##     has_scale: function
##     input: function
##     n: function
##     non_position_scales: function
##     scales: list
##     super:  <ggproto object: Class ScalesList, gg> 
##   ..$ mapping    :List of 2
##   .. ..$ x: symbol mpg
##   .. ..$ y: symbol wt
##   ..$ theme      : list()
##   ..$ coordinates:Classes 'CoordCartesian', 'Coord', 'ggproto', 'gg' <ggproto object: Class CoordCartesian, Coord, gg>
##     aspect: function
##     distance: function
##     expand: TRUE
##     is_linear: function
##     labels: function
##     limits: list
##     modify_scales: function
##     range: function
##     render_axis_h: function
##     render_axis_v: function
##     render_bg: function
##     render_fg: function
##     setup_data: function
##     setup_layout: function
##     setup_panel_params: function
##     setup_params: function
##     transform: function
##     super:  <ggproto object: Class CoordCartesian, Coord, gg> 
##   ..$ facet      :Classes 'FacetWrap', 'Facet', 'ggproto', 'gg' <ggproto object: Class FacetWrap, Facet, gg>
##     compute_layout: function
##     draw_back: function
##     draw_front: function
##     draw_labels: function
##     draw_panels: function
##     finish_data: function
##     init_scales: function
##     map_data: function
##     params: list
##     setup_data: function
##     setup_params: function
##     shrink: TRUE
##     train_scales: function
##     vars: function
##     super:  <ggproto object: Class FacetWrap, Facet, gg> 
##   ..$ plot_env   :<environment: R_GlobalEnv> 
##   ..$ labels     :List of 3
##   .. ..$ x     : chr "mpg"
##   .. ..$ y     : chr "wt"
##   .. ..$ colour: chr "cyl"
##   ..- attr(*, "class")= chr [1:2] "gg" "ggplot"
##  - attr(*, "class")= chr "ggplot_built"

As I started this project, I became frustrated trying to understand/navigate through the nested list-like structure of ggplot objects. As you can imagine, it isn’t an optimal approach to print out the structure everytime you want to checkout a particular element. Out of this frustration came an idea to build this tool to help interact with and visualize this structure. Thankfully, my wonderful GSoC mentor Toby Dylan Hocking agreed that this project could bring value to the ggplot2 community and encouraged me to pursue it.

By default, this tool presents a radial Reingold–Tilford Tree of this nested list structure, but also has options to use the collapsable or cartesian versions. It also leverages the shinyAce package which allows users to send arbitrary ggplot2 code to a shiny server thats evaluate the results and re-renders the visuals. I’m quite happy with the results as I think this tool is a great way to quickly grasp the internal building blocks of ggplot(s). Please share your thoughts below!