Adding Triangles to a ggplot2 Colorbar in R: A Custom Solution for Enhanced User Experience

Adding Triangles to a ggplot2 Colorbar in R

As of my knowledge cutoff in December 2023, creating custom colorbars with triangles indicating out-of-bounds values in ggplot2 is not a straightforward process. However, it’s possible to achieve this by extending the existing guide_colourbar functionality and creating a new guide class.

Why Use Custom Colorbars?

Colorbars are an essential component of ggplot2 plots, providing visual cues for users to interpret data values. By adding triangles to indicate out-of-bounds values, we can enhance the user experience and provide more meaningful information about the data.

Overview of Existing Code

The existing code in the Stack Overflow question provides a starting point for creating custom colorbars with triangles. The my_triangle_colourbar function extends the guide_colourbar functionality by adding two triangles at the end of the colorbar.

Custom Guide Class

To create a custom guide class, we need to extend the guide class and define a new method called guide_gengrob. This method will be responsible for generating the custom guide elements.

## Step 1: Define the Custom Guide Class
library(ggplot2)
library(gtable)

my_triangle_colourbar <- function(...) {
  # Create an instance of the guide class
  guide <- guide_colourbar(...)
  
  # Add our own class to the guide
  class(guide) <- c("my_triangle_colourbar", class(guide))
  
  return(guide)
}

Custom Guide Elements

To generate custom guide elements, we need to define a new method called guide_gengrob. This method will be responsible for drawing the triangles and other elements that make up our custom colorbar.

## Step 2: Define the guide_gengrob Method
guide_gengrob <- function(...) {
  # First, draw the normal colourbar
  guide <- NextMethod()
  
  # Extract the bar / colours
  is_bar <- grep("^bar$", guide$layout$name)
  bar <- guide$grobs[[is_bar]]
  extremes <- c(bar$raster[1], bar$raster[length(bar$raster)])
  
  # Extract the size
  width  <- guide$widths[guide$layout$l[is_bar]]
  height <- guide$heights[guide$layout$t[is_bar]]
  short  <- min(convertUnit(width, "cm", valueOnly = TRUE),
                 convertUnit(height, "cm", valueOnly = TRUE))
  
  # Make space for triangles
  guide <- gtable_add_rows(guide, unit(short, "cm"),
                           guide$layout$t[is_bar] - 1)
  guide <- gtable_add_rows(guide, unit(short, "cm"),
                           guide$layout$t[is_bar])
  
  # Draw triangles
  top <- polygonGrob(
    x = unit(c(0, 0.5, 1), "npc"),
    y = unit(c(0, 1, 0), "npc"),
    gp = gpar(fill = extremes[1], col = NA)
  )
  bottom <- polygonGrob(
    x = unit(c(0, 0.5, 1), "npc"),
    y = unit(c(1, 0, 1), "npc"),
    gp = gpar(fill = extremes[2], col = NA)
  )
  
  # Add triangles to the guide
  guide <- gtable_add_grob(
    guide, top,
    t = guide$layout$t[is_bar] - 1,
    l = guide$layout$l[is_bar]
  )
  guide <- gtable_add_grob(
    guide, bottom,
    t = guide$layout$t[is_bar] + 1,
    l = guide$layout$l[is_bar]
  )
  
  return(guide)
}

Using the Custom Guide

Now that we have defined our custom guide class and method, we can use it in a scale.

## Step 3: Use the Custom Guide
g <- ggplot(mtcars, aes(mpg, wt)) +
  geom_point(aes(colour = drat))

g + scale_colour_viridis_c(
    limits = c(3, 4), oob = scales::oob_squish,
    guide = my_triangle_colourbar()
)

This will create a custom colorbar with triangles indicating out-of-bounds values.

Conclusion

Creating custom colorbars with triangles is possible in ggplot2 by extending the existing guide_colourbar functionality and defining a new method called guide_gengrob. This allows us to add our own custom elements, such as triangles, to provide more meaningful information about the data.

Last modified on 2023-11-10