Imagine I have a data matrix, my_matrix, that looks like this:

On a single plot, I wanted to create a line for Sample A (connecting the values for variables 1, 2, and 3), a line for Sample B, and so on.

I melted the data and plotted it like so:

ggplot(data=melt(my_mtrx), 
       aes(x=Var2, y=value, colour=Var1)) +
  geom_line() +
  scale_color_manual(values = c(rep("green", 105)))

The issue I’m facing is that in the legend, ggplot creates an entry for each of Sample A, B, and C. But imagine that Sample A and C actually belong to one ‘group’, and C to another. So instead of a legend like this:

Sample A
Sample B
Sample C

I want a legend like this:

Group X
Group N