Plot a legend outside of the plotting area in base graphics?

Question

As the title says: How can I plot a legend outside the plotting area when using base graphics?

I thought about fiddling around with layout

Answer 1

I like to do it like this:

par(oma=c(0, 0, 0, 5))
plot(1:3, rnorm(3), pch=1, lty=1, type="o", ylim=c(-2,2))
lines(1:3, rnorm(3), pch=2, lty=2, type="o")
legend(par('usr')[2], par('usr')[4], bty='n', xpd=NA,
       c("group A", "group B"), pch=c(1, 2), lty=c(1,2))

The only tweaking required is in setting the right margin to be wide enough to accommodate the legend.

However, this can also be automated:

dev.off() # to reset the graphics pars to defaults
par(mar=c(par('mar')[1:3], 0)) # optional, removes extraneous right inner margin space
plot.new()
l <- legend(0, 0, bty='n', c("group A", "group B"), 
            plot=FALSE, pch=c(1, 2), lty=c(1, 2))
# calculate right margin width in ndc
w <- grconvertX(l$rect$w, to='ndc') - grconvertX(0, to='ndc')
par(omd=c(0, 1-w, 0, 1))
plot(1:3, rnorm(3), pch=1, lty=1, type="o", ylim=c(-2, 2))
lines(1:3, rnorm(3), pch=2, lty=2, type="o")
legend(par('usr')[2], par('usr')[4], bty='n', xpd=NA,
       c("group A", "group B"), pch=c(1, 2), lty=c(1, 2))

Answer 2

Recently I found very easy and interesting function to print legend outside of the plot area where you want.

Make the outer margin at the right side of the plot.

par(xpd=T, mar=par()$mar+c(0,0,0,5))

Create a plot

plot(1:3, rnorm(3), pch = 1, lty = 1, type = "o", ylim=c(-2,2))
lines(1:3, rnorm(3), pch = 2, lty = 2, type="o")

Add legend and just use locator(1) function as like below. Then you have to just click where you want after load following script.

legend(locator(1),c("group A", "group B"), pch = c(1,2), lty = c(1,2))

Try it

Answer 3

Maybe what you need is par(xpd=TRUE) to enable things to be drawn outside the plot region. So if you do the main plot with bty='L' you'll have some space on the right for a legend. Normally this would get clipped to the plot region, but do par(xpd=TRUE) and with a bit of adjustment you can get a legend as far right as it can go:

 set.seed(1) # just to get the same random numbers
 par(xpd=FALSE) # this is usually the default

 plot(1:3, rnorm(3), pch = 1, lty = 1, type = "o", ylim=c(-2,2), bty='L')
 # this legend gets clipped:
 legend(2.8,0,c("group A", "group B"), pch = c(1,2), lty = c(1,2))

 # so turn off clipping:
 par(xpd=TRUE)
 legend(2.8,-1,c("group A", "group B"), pch = c(1,2), lty = c(1,2))

Answer 4

Adding another simple alternative that is quite elegant in my opinion.

Your plot:

plot(1:3, rnorm(3), pch = 1, lty = 1, type = "o", ylim=c(-2,2))
lines(1:3, rnorm(3), pch = 2, lty = 2, type="o")

Legend:

legend("bottomright", c("group A", "group B"), pch=c(1,2), lty=c(1,2),
       inset=c(0,1), xpd=TRUE, horiz=TRUE, bty="n"
       )

Result:

Here only the second line of the legend was added to your example. In turn:

• inset=c(0,1) - moves the legend by fraction of plot region in (x,y) directions. In this case the legend is at "bottomright" position. It is moved by 0 plotting regions in x direction (so stays at "right") and by 1 plotting region in y direction (from bottom to top). And it so happens that it appears right above the plot.
• xpd=TRUE - let's the legend appear outside of plotting region.
• horiz=TRUE - instructs to produce a horizontal legend.
• bty="n" - a style detail to get rid of legend bounding box.

Same applies when adding legend to the side:

par(mar=c(5,4,2,6))
plot(1:3, rnorm(3), pch = 1, lty = 1, type = "o", ylim=c(-2,2))
lines(1:3, rnorm(3), pch = 2, lty = 2, type="o")

legend("topleft", c("group A", "group B"), pch=c(1,2), lty=c(1,2),
       inset=c(1,0), xpd=TRUE, bty="n"
       )

Here we simply adjusted legend positions and added additional margin space to the right side of the plot. Result:

Answer 5

No one has mentioned using negative inset values for legend. Here is an example, where the legend is to the right of the plot, aligned to the top (using keyword "topright").

# Random data to plot:
A <- data.frame(x=rnorm(100, 20, 2), y=rnorm(100, 20, 2))
B <- data.frame(x=rnorm(100, 21, 1), y=rnorm(100, 21, 1))

# Add extra space to right of plot area; change clipping to figure
par(mar=c(5.1, 4.1, 4.1, 8.1), xpd=TRUE)

# Plot both groups
plot(y ~ x, A, ylim=range(c(A$y, B$y)), xlim=range(c(A$x, B$x)), pch=1,
               main="Scatter plot of two groups")
points(y ~ x, B, pch=3)

# Add legend to top right, outside plot region
legend("topright", inset=c(-0.2,0), legend=c("A","B"), pch=c(1,3), title="Group")

The first value of inset=c(-0.2,0) might need adjusting based on the width of the legend.

Answer 6

I can offer only an example of the layout solution already pointed out.

layout(matrix(c(1,2), nrow = 1), widths = c(0.7, 0.3))
par(mar = c(5, 4, 4, 2) + 0.1)
plot(1:3, rnorm(3), pch = 1, lty = 1, type = "o", ylim=c(-2,2))
lines(1:3, rnorm(3), pch = 2, lty = 2, type="o")
par(mar = c(5, 0, 4, 2) + 0.1)
plot(1:3, rnorm(3), pch = 1, lty = 1, ylim=c(-2,2), type = "n", axes = FALSE, ann = FALSE)
legend(1, 1, c("group A", "group B"), pch = c(1,2), lty = c(1,2))