Allow to set na.color transparent

[billy34]

It would be good to be able to set na.color to NA in order to not display NA values (be transparent)

I managed to do this be changing a single line in ScaleColorPlane.R (near line 72)

# prefill with the NA color so we can do the projection as an inset, process
# the NA color to ensure proper #xxxxxx form regardless of unput type
df[[aesthetics[1]]] <- grDevices::rgb(
  t(grDevices::col2rgb(self$na.color[1])),
  maxColorValue = 255)

to

# prefill with the NA color so we can do the projection as an inset, process
# the NA color to ensure proper #xxxxxx form regardless of unput type
df[[aesthetics[1]]] <- do.call(grDevices::rgb,
  c(as.list(grDevices::col2rgb(self$na.color[1],TRUE)),
  maxColorValue = 255))

I had to use do.call because rgb does not allow to provide an RGBA vector to the first parameter (only RGB)

[wmurphyrd]

Good find. Could you send a pr?