r - Flip ordering of legend without altering ordering in plot

Question

I have found that when adding coord_flip() to certain plots using ggplot2 that the order of values in the legend no longer lines up with the order of values in the plot.

For example:

dTbl = data.frame(x=c(1,2,3,4,5,6,7,8),
                  y=c('a','a','b','b','a','a','b','b'),
                  z=c('q','q','q','q','r','r','r','r'))

print(ggplot(dTbl, aes(x=factor(y),y=x, fill=z)) +
      geom_bar(position=position_dodge(), stat='identity') +
      coord_flip() +
      theme(legend.position='top', legend.direction='vertical'))

I would like the 'q' and 'r' in the legend to be reversed without changing the order of 'q' and 'r' in the plot.

scale.x.reverse() looked promising, but it doesn't seem to work within factors (as is the case for this bar plot).

Answer 1

You're looking for guides:

ggplot(dTbl, aes(x=factor(y),y=x, fill=z)) +
      geom_bar(position=position_dodge(), stat='identity') +
      coord_flip() +
      theme(legend.position='top', legend.direction='vertical') + 
      guides(fill = guide_legend(reverse = TRUE))

I was reminded in chat by Brian that there is a more general way to do this for arbitrary orderings, by setting the breaks argument:

ggplot(dTbl, aes(x=factor(y),y=x, fill=z)) +
      geom_bar(position=position_dodge(), stat='identity') +
      coord_flip() +
      theme(legend.position='top', legend.direction='vertical') + 
      scale_fill_discrete(breaks = c("r","q"))