Question

When I run this code

plot(c(0,1), c(0, 1), type = "n")
legend("topleft", legend = c("Model", "Data"),
           lwd = c(3, NA),
           pch = c(NA, 16),
           bty = "n", inset = 0.02,
           cex = 2)

as expected, I see this:

However, when I wrap it in pdf("legendTest.pdf") … dev.off(), the saved PDF has a line through the point.

Can I correct this, or do I need to just save in another format? I’m on a Mac, and the problem is present viewing the PDF using Preview and Chrome. I downloaded Adobe Reader to see what it did, and then the legend text and pch point disappeared, leaving only the model line.

Answer 1

You can fix this by explicitly setting the line type (argument lty), like this:

plot(c(0,1), c(0, 1), type = "n")
legend("topleft", legend = c("Model", "Data"),
       lwd = c(3, NA),
       lty = c(1, 0), # 0=blank, 1=solid (default). See ?par for more.
       pch = c(NA, 16),
       bty = "n", inset = 0.02,
       cex = 2)

EDIT

The documentation for the lwd argument, in ?par explains that:

‘lwd’ The line width, a positive number, defaulting to ‘1’. The
interpretation is device-specific, and some devices do not
implement line widths less than one.

Mac’s default R graphical device must be one of those. (FWIW, with your code, my own Windows graphics device does show a line through the data point, just as in the pdf.)

In any case, it looks like it’s generally safer to use lty than lwd to control this particular detail.