hadley/ggplot2 — Issue #410: NAs in colored scatter plots are misleading
Author: ifellows
Created: 2012-02-21 18:27:28+00:00
State: open

Consider:

dat <- data.frame(a = 1:10,b=1:10,c=c(1:9,NA))
ggplot() +
geom_point(aes(x = a,y = b,colour = c),data=dat) +
scale_colour_gradient(guide = guide_legend(),low = '#000000',high = '#e3e1e2')

There is no indication in the legend what NA values look like, and even if there was, the reader would not be able to distinguish it from the color scale. It seems to me that NAs need either a special plotting symbol (perhaps an open point with "NA" written inside it), or they should be dropped with a warning.