Different legend-keys inside same legend in ggplot2

Question

Let\'s say I don\'t need a \'proper\' variable mapping but still would like to have legend keys to help the chart understanding. My actual data are similar to the following

Answer 1

You can use override.aes= inside guides() function to change default appearance of legend. In this case your guide is color= and then you should set shape=c(NA,16) to remove shape for line and then linetype=c(1,0) to remove line from point.

ggplot(df) +
  geom_line(aes(id, line, colour = "line")) +
  geom_point(aes(id, points, colour = "points"))+
  guides(color=guide_legend(override.aes=list(shape=c(NA,16),linetype=c(1,0))))

Answer 2

I am not aware of any way to do this easily, but you can do a hack version like this (using your melted dataframe):

p <- ggplot(df.m, aes(id, value)) +
  geom_line(aes(colour = variable, linetype = variable)) + scale_linetype_manual(values = c(1,0)) +
  geom_point(aes(colour = variable, alpha = variable)) + scale_alpha_manual(values = c(0,1))

The key is that you need to get the mapping right to have it displayed correctly in the legend. In this case, getting it 'right', means fooling it to look the way you want it to. It's probably worth pointing out this only works because you can set linetype to blank (0) and then use the alpha scale for the points. You can't use alpha for both, because it will only take one scale.