Calculating the appropriate inset value for legends automatically

Question

Is it possible to obtain the appropriate value of inset automatically so that the left corner of legend will always be just outside of the top right corner of plot?

In the plot below I had to try several values for inset manually. It would be nice to not have to do it manually since I have to make multiple plots.

graphics.off()
windows(width = 5, height = 5)
set.seed(42)
par(mar = c(5,5,1,10))
plot(rnorm(50,15,5), rnorm(50,15,3),
                xlim = c(0,30), ylim = c(5,25),
                pch = 19, col = c("red","blue"))

par(xpd = TRUE)
legend("topright", inset = c(-.80, 0),
                pch = 19, col = c("red","blue"),
                legend = c("LEGEND 1","Second Legend"))

Answer 1

After the plot call, before adding the legend, use par("usr")* to extract the coordinates of the plotting region.

Then, instead of positioning the legend using a 'keyword' and inset, use x and y with the top-right coordinates of the plotting region obtained from par("usr"). Adjust x with a suitable coefficient.

coord <- par("usr")
legend(x = coord[2] * 1.05, y = coord[4],
       pch = 19, col = c("red", "blue"),
       legend = c("LEGEND 1", "Second Legend"))

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And just for fun, a more convoluted alternative.

After plotting, call legend with position topright, but without plotting it to the device (plot = FALSE), and assign it to an object.

Extract the left x and the top y coordinate of the legend box, and its width (see Value section in ?legend), to be used in x and y in legend:

leg <- legend("topright", pch = 19, col = c("red", "blue"),
              legend = c("LEGEND 1", "Second Legend"),
              plot = FALSE)

legend(x = (leg$rect$left + leg$rect$w) * 1.05, y = leg$rect$top,
       pch = 19, col = c("red", "blue"),
       legend = c("LEGEND 1", "Second Legend"))

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*From ?par

usr A vector of the form c(x1, x2, y1, y2) giving the extremes of the user coordinates of the plotting region.

The position calculations that are made when inset parameter(s) have been specified, are in fact based on par("usr") (see line 188-199 in the legend code).