r - 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 and produce an empty plot to only contain the legend, but I would be interested in a way using just the base graph facilities and e.g., par(mar = ) to get some space on the right of the plot for the legend.

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Here an example:

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

produces:

But as said, I would like the legend to be outside the plotting area (e.g., to the right of the graph/plot.

Answer 1

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.