Border for line in geom_step in ggplot2

Question

I haven't asked a question using this before so forgive me if I haven't provided what's needed/explained in sufficient detail. I'm using geom_step in ggplot2 to try and produce step curves for data with two separate two-level factors (coded as a single four-level factor). I'd like the first factor (in the below, A vs B) to be represented by colour and the second factor (in the below, 1 vs 2) to be represented by line fill, i.e filled vs white. Here is an edited mock-up of what I'm looking for:

Is it possible to add a border to lines constructed by geom_step in ggplot2?

If not, could I overlay a second geom_step line with smaller line width and different colours to "manually" add the line fill? I've tried adding another geom_step with different colours in the scale_colour_manual term, but this simply duplicates the first geom_step curve and returns the message "Scale for 'colour' is already present. Adding another scale for 'colour', which will replace the existing scale."

Example code is below.

events <- rep(c(0,1,2,3,4),4)
individual <- (as.factor(rep(c("A1","A2","B1","B2"),each=5)))
step <- c(1,2,3,4,5,3,4,5,6,7,5,6,7,8,9,7,8,9,10,11)
df <- data.frame(events,individual,step)

ggplot(df, aes(x=events, group=individual, colour=individual, y=step)) + 
  geom_step(size=1.8) +
  scale_colour_manual(values=c("green2","green2", "orange", "orange"), name="Individual")

Answer 1

This isn't quite a full solution:

It's fairly easy to add a white strip by adding another geom_step command:

events<-rep(c(0,1,2,3,4),4)
individual<-(as.factor(rep(c("A1","A2","B1","B2"),each=5)))
step<-c(1,2,3,4,5,3,4,5,6,7,5,6,7,8,9,7,8,9,10,11)
filled <- rep(c(FALSE,TRUE),each=5)

data_frame<-data.frame(events,individual,step,filled)
ggplot(data_frame, aes(x=events, group=individual, colour=individual, y=step)) + 
geom_step(size=1.8) +
scale_colour_manual(values=c("green2","green2", "orange", "orange"), name="Individual") +
geom_step(data=df[df$filled,], size=1.1, colour="white")

Unfortunately I'm not sure how to make the legend consistent with this.

An easier alternative would be to make some lines lighter. You can define a lighten_colour function:

lighten_colour <- function(colour, lightness=0.5) {
    # lightness should be between 0 and 1
    white <- col2rgb("white") / 255
    colour <- col2rgb(colour) / 255
    rgb(t(white*lightness + colour*(1-lightness)))
}

... and use this for the plot colours.

ggplot(data_frame, aes(x=events, group=individual, colour=individual, y=step)) + 
geom_step(size=1.8) +
scale_colour_manual(values=c(lighten_colour("green2"),
                             "green2",
                             lighten_colour("orange"),
                             "orange"),
                    name="Individual")