ggplot2: One legend with two visual properties derived from common variable

9
votes

How can I get a single legend that captures both colour and size?

I was under the impression that a common legend is default if a common variable is used, but the following example shows I am missing something.

library(ggplot2)

input <- as.data.frame(matrix(runif(60),nrow=20,ncol=3))
colnames(input) <- c("A","B","C")

p <- ggplot(input,aes(A,B,size=C,color=C)) + geom_point()

enter image description here

Thanks to Arun for a comment that prompted this edit. So, if one just uses size (and forgets color) one gets a legend that depicts three sizes but many more sizes are depicted in the plot.

enter image description here

So what I would be after is similar behaviour - a legend that shows some values of the common variable and depicts the corresponding sizes and colors.

2
rggplot2legend
you seem to be plotting the aesthetic size and color as continuous attribute (C is not a factor). How can you combine the legend when it isn't discrete? try ggplot(input, aes(A, B)) + geom_point(aes(size = factor(C), color = factor(C))) (you'll see a huge combined legend) - Arun
Thanks @Arun. Size and color are both continuous. I appreciate that size cannot have a continuous legend representation, and I assumed it would just sample the size at intervals, which could allow different sizes to appear the in the plot that just those in the legend. - Rahul Savani
@Arun: After all, it already does for size: The legend does not display all sizes that appear in the plot. Is it unreasonable to want a subset of the common variable to appear in the legend, depicted with the corresponding color and size? - Rahul Savani
Um.. I understand your point. But with continuous range of colours, I guess it becomes not possible. - Arun

2 Answers

10
votes

The colorbar cannot be merged, but a normal legend can,

p + guides(colour = guide_legend())
0
votes

I needed to make the labels for size and color the same, and make sure it's working with the same information combined with the guides line.

p+geom_jitter(data=df, aes(x=x, y=y, color=value, size = value)) 
+scale_size_continuous(name = "Legend Name", breaks= c(.25, .50,.75), labels=c(".25",".50",".75"))+scale_colour_gradient(name = "Legend Name", breaks= c(.25, .50,.75), labels=c(".25", ".50",".75"))+ 
guides(colour = guide_legend())