R help - Dec 2019 - Adding a legend to a (multi-facet) plot produced by ggplot().

How about defining your dataset differently, making the colouring property a
variable?

xxx <- data.frame(x=rep(x, 4), y=c(y2, y3),
grp=factor(rep(c("a","b"),each=20, times=2)),
type=factor(rep(c("clyde", "irving"), each=40)))
ggplot(xxx, aes(x,y, colour=type, shape=type)) + geom_point() +
geom_abline(intercept=3, slope=2) + facet_wrap(vars(grp)) +
scale_colour_manual(values=c("blue", "red"))  +
scale_shape_manual(values=c(20,3))

Then you could also plot the four groups separately if you wanted to:

ggplot(xxx, aes(x,y, colour=type, shape=type)) + geom_point() +
geom_abline(intercept=3, slope=2) + facet_grid(rows=vars(type), cols=vars(grp))
+ scale_colour_manual(values=c("blue", "red"))  +
scale_shape_manual(values=c(20,3))

Antony Unwin
University of Augsburg, 
Germany



> From: Rolf Turner <r.turner at auckland.ac.nz>
> Subject: [R] Adding a legend to a (multi-facet) plot produced by ggplot().
> Date: 1 December 2019 at 01:04:46 CET
> To: R help <R-help at r-project.org>
> 
> 
> 
> I have been struggling to add a legend as indicated in the subject line,
> with no success at all.  I find the help to be completely bewildering.
> 
> I have attached the code of what I have tried in the context of a simple
> reproducible example.
> 
> I have also attached a pdf file of a plot produced with base graphics to
illustrate roughly what I am after.
> 
> I would be grateful if someone could point me in the right direction.
> 
> cheers,
> 
> Rolf Turner
> 
> -- 
> Honorary Research Fellow
> Department of Statistics
> University of Auckland
> Phone: +64-9-373-7599 ext. 88276