How to reorder overlaying order of geoms but keep legend order intact in ggplot

Question

I know I can switch the line plotting order (i.e., which line is plotted 1st, 2nd, 3rd,...) by changing the levels of a grouping factor. However, doing so also switches the order of the ggplot legend.

How do I change the plotting order but retain the original legend order?

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Example

Given the following data.frame:

dat <- data.frame(id = rep(factor(letters[1:3]),3), y = c(1:3,3,2,1,1,3,1), x = rep(1:3,each = 3))

I can switch the line plotting order by changing the levels of the id factor:

• e.g., by reordering the levels using dat$id = relevel(dat$id, 'c').

Using the following ggplot code for the original and relevel'ed data:

library(ggplot2)

#Create coloring objects to kep color consistent:
  cols <- rep(1:3,3)
    names(cols) <- letters[1:3]

#Create line graph:
  ggplot(dat,aes(x=x,y=y,color=id)) + geom_line(size = 2) + 
    scale_colour_manual('id',values=cols)  ##set custom static coloring

produces the following 2 graphs:

^{Left: original level order for factor id | Right: using relevel}

Notice that the lines have been successfully reordered: line c goes from being printed last (and therefore on top) to being printed first (and therefore on the bottom).

However, the legend has also switched order!

How do I retain the legend order from the left graph but modify the line printing order to match that of the right graph?

Answer 1

You will probably have found a solution by now. Just saw this question is still 'unanswered' and here a suggestion, based on @Hendrik's comment - which was very helpful but not the entire solution (his code gives your plot 2)

dat$id2 = relevel(dat$id, 'c') # just create a dummy column with the releveled factors

ggplot(dat) + 
  geom_line(aes(x = x, y = y, color = id2), size = 2) + 
# use your dummy column for the line order
  scale_colour_manual('id',values = cols, breaks = levels(dat$id))  
# use @Hendriks break suggestion