ggplot2: how to create correct legend after using scale_xx_manual

Question

I have a plot with three different lines. I want one of those lines to have points on as well. I also want the two lines without points to be thicker than the one without points. I have managed to get the plot I want, but I the legend isn't keeping up.

library(ggplot2)

y <- c(1:10, 2:11, 3:12)
x <- c(1:10, 1:10, 1:10)
testnames <- c(rep('mod1', 10), rep('mod2', 10), rep('meas', 10))
df <- data.frame(testnames, y, x)

ggplot(data=df, aes(x=x, y=y, colour=testnames)) +
   geom_line(aes(size=testnames)) +
   scale_size_manual("", values=c(0.5,1,1)) +
   geom_point(aes(alpha=testnames), size=5, shape=4) +
   scale_alpha_manual("", values=c(1, 0, 0))

I can remove the second (black) legend:

ggplot(data = df, aes(x=x, y=y, colour=testnames)) +
   geom_line(aes(size=testnames)) +
   scale_size_manual("", values=c(0.5,1,1), guide='none') +
   geom_point(aes(alpha=testnames), size=5, shape=4) +
   scale_alpha_manual("", values=c(1, 0.05, 0.05), guide='none')

But what I really want is a merge of the two legends - a legend with colours, cross only on the first variable (meas) and the lines of mod1 and mod2 thicker than the first line. I have tried guide and override, but with little luck.

Answer 1

You don't need transparency to hide the shapes for mod1 and mod2. You can omit these points from the plot and legend by setting their shape to NA in scale_shape_manual:

ggplot(data = df, aes(x = x, y = y, colour = testnames, size = testnames)) +
  geom_line() +
  geom_point(aes(shape = testnames), size = 5) +
  scale_size_manual(values=c(0.5, 2, 2)) +
  scale_shape_manual(values=c(8, NA, NA)) 

This gives the following plot:

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NOTE: I used some more distinct values in the size-scale and another shape in order to better illustrate the effect.