Avoid vlines groups to appear in legend

Question

I want to make a line plot with custom colors and additional vlines with custom colors too.

It all works well except I cannot find a way of removing the vline groups from the legend... after checking many questions, all I could get is show.legend=FALSE but that does not the trick completely.

This is my MWE:

datn <- read.table(header=TRUE, text='
supp dose length
  OJ  0.5  13.23
  OJ  1.0  22.70
  OJ  2.0  26.06
  VC  0.5   7.98
  VC  1.0  16.77
  VC  2.0  26.14
')
datn$supp <- factor(datn$supp, levels=unique(datn$supp))
plot_palette <- c("indianred1","darkgoldenrod1")
vlines <- data.frame(xint = c(1,1.25,1.75), grp = LETTERS[1:3], clr = c("lightskyblue1","cadetblue2","cyan4"))
plot_palette <- c(plot_palette, vlines$clr)
#
P <- ggplot2::ggplot(datn, ggplot2::aes(x=dose, y=length, color=supp)) +
  ggplot2::geom_line(linetype = 1, linewidth = 1.75) +
  ggplot2::geom_vline(data=vlines, ggplot2::aes(xintercept=xint, color=grp), linetype=4, linewidth=2.25, show.legend = FALSE) +
  ggplot2::scale_color_manual(values=plot_palette) +
  ggplot2::theme_light()
P
grDevices::dev.off()

which produces:

For clarity, all I want to do is to remove groups A, B, C from the legend.

I do not even understand why they show there in the first place, since they are not supp levels and I explicitly specify show.legend=FALSE for geom_vline... anyone can explain?

Answer 1

You could use the breaks= argument to set the values which should be displayed in the legend. However, to still color all lines you have to set the limits= or use a named vector for the color palette passed to values= as I do below:

library(ggplot2)

names(plot_palette) <- c(
  levels(datn$supp), vlines$grp
)

ggplot(datn, aes(x = dose, y = length, color = supp)) +
  geom_line(linetype = 1, linewidth = 1.75) +
  geom_vline(
    data = vlines, aes(xintercept = xint, color = grp),
    linetype = 4, linewidth = 2.25, show.legend = FALSE
  ) +
  scale_color_manual(
    values = plot_palette,
    breaks = levels(datn$supp)
  ) +
  theme_light()