gradient fill violin plots using ggplot2

Question

I want to gradient fill a violin plot based on the density of points in the bins (blue for highest density and red for lowest).

I have generated a plot using the following commands but failed to color it based on density (in this case the width of the violin. I also would like to generate box plots with similar coloring).

library("ggplot2")
data(diamonds)

ggplot(diamonds, aes(x=cut,y=carat)) + geom_violin()

Can you show a picture (drawing?) of your desired output? Why do you need the colors; doesn't the width of the violin already implicate density? — Heroka
Hi Heroka, actually my final goal is to color the violin plot ( or the box plot which I prefer in my case due to space restrictions) based on another parameter. So I want to show how the distribution of the let's say carats affect another variable; let say abundance. For example If diamonds with high carats are rare (just an assumption) then the violin plot region with larger width should be more blue and the narrow regions will be more reddish. — user2438149
For the diamonds data set will it be possible to the fill the cut vs carats violin pots with price variable to show the relationship between the carat and the price. Thanks! — user2438149
ggplot does not support the filling of individual graphic objects with gradients. While it would certainly be possible to make the solid coloring of each box plot dependent on some variable, you can't create a gradient within an individual box. There are probably ways to "hack around" the limitation and draw something that resembles a box plot with a gradient fill, but it'd be quite complex and defeats the point of using ggplot for simplicity. — jdobres
Does this answer your question? How to assign a Colour Scale based on Y-axis in ggplot — tjebo

Derek Corcoran Derek Corcoran · Accepted Answer · 2016-12-11T15:25:52

to change the colour of the violin plot you use fill = variable, like this:

ggplot(diamonds, aes(x=cut,y=carat)) + geom_violin(aes(fill=cut))

same goes for boxplot

ggplot(diamonds, aes(x=cut,y=carat)) + geom_boxplot(aes(fill=cut))

but whatever value you have has to have the same value for each cut, that is, if you wanted to use for example mean depth/cut as the color variable you would have to code it.

with dplyr group your diamonds by cut and with summarize get the mean depth (or any other variable)

library(dplyr)
diamonds_group <- group_by(diamonds, cut)
diamonds_group <- summarize(diamonds_group, Mean_Price = mean(price))

Then I used diamonds2 as a copy of diamonds to then manipulate the dataset

diamonds2 <- diamonds

I merge both dataframes to get the Mean_Depth as a variable in diamonds2

diamonds2 <- merge(diamonds2, diamonds_group)

And now I can plot it with mean depth as a color variable

ggplot(diamonds2, aes(x=cut,y=carat)) + geom_boxplot(aes(fill=Mean_Price)) + scale_fill_gradient2(midpoint = mean(diamonds2$price))

gradient fill violin plots using ggplot2

2 Answers