Combine and merge legends in ggplot2 with patchwork

Question

I would like to combine two or more plots merging their legends.

For example, I can create some data and two scenarios as follows.

# packages
library(ggplot2)
library(patchwork)

# first plot
set.seed(07042020)
x <- runif(50)
y <- runif(50)
data1 <- data.frame(x = x, y = y, z = runif(50, 0, 2))
p1 <- ggplot(data1) + geom_point(aes(x, y, col = z))
p1

data2 <- data.frame(x = x, y = y, z = runif(50, -1, 1))
p2 <- ggplot(data2) + geom_point(aes(x, y, col = z))
p2

The following code is what I tried so far but it's not the intended result. I would like to merge the two plots with a single legend, i.e. create a unique and common legend "z" in such a way that the points of the two plots are coloured according to this common legend. Is this possible?

p1 + p2 + plot_layout(guides = "collect")

^{Created on 2020-04-07 by the reprex package (v0.3.0)}

Answer 1

I think two legends can only be combined when they have the exact same properties, i.e. share limits, titles, labels, breaks etc. You can provide a common legend by sharing a common scale, one way to do that in patchwork is to use the & operator, which sort of means 'apply this to all previous plots':

p1 + p2 + plot_layout(guides = "collect") & 
  scale_colour_continuous(limits = range(c(data1$z, data2$z)))

Only downside is that you'd probably manually have to specify the limits as the scale in p1 does not know about the values in p2.

EDIT: I've since learned that legends are merged if they are the same on the level of {grid} objects. As a consequence, you probably cannot merge legends that are styled differently through the theme or if keys represent a different number of layers (i.e. when combining a plot with 1 point layer with a plot with 2 point layers represented in the legend).