ggplot2 custom legend shapes

Question

When we have both shape and color legend in scatter plot, the shape of color legend is misleading:

foo <- data.frame(
  length=runif(10),
  height=runif(10),
  group=as.factor(sample(3,10,rep=T)),
  quality=as.factor(sample(2,10,rep=T))
)

ggplot(foo, aes(x = length, y = height, color=group, shape=quality)) + 
  geom_point(size=5)

This will produce the plot below. As you see, the "circle" shape is reserved for the objects of quality==1, however in the group legend, all 3 groups are presented in "circle" shapes - with different colors, this can be misleading.

It was too much better if the group legend was represented by a shape not already reserved for a specific purpose, like just to fill the whole legend item with the specific color.

Do you have any simple idea how to solve this?

Answer 1

It is possible to manually change the properties of the legend using guides:

ggplot(foo, aes(x = length, y = height, color=group, shape=quality)) + 
  geom_point(size=5) + 
  guides(colour = guide_legend(override.aes = list(shape = 15)))

Just play around with the shape paramater to find an appropriate shape.