It is not:

  • anything in the aes of the ggplot call
  • color=FALSE
  • color=NULL

It is:

  • color=NA in the geom_density call

e.g.

ggplot(data, aes(x=value, fill=grouping))+geom_density(<b>color=NA</b>)

Example (this is some real data I'm currently working with, but I've changed …