R inconsistency: why add=T sometimes works and sometimes not in the plot() function?

Question

Why is R inconsistent with the add parameter in the plot() function? It sometimes works and sometimes doesn\'t! In this example, it takes the param

Answer 1

This is because when you call plot(0:10, 0:10*3) or plot(c(2, 3, 4), c(20,10,15)), you are indirectly calling plot.default(), which in turn calls plot.xy(), whereas the other two calls you mention are running plot.function(). add is an argument for plot.function(), but not for plot.xy().

You can get around this inconsistency by setting par(new = TRUE), but then you need to make sure that you don't add fresh axis labels or redraw the axes. EDIT: As pointed out in the comment, you have to make sure that the range is the same as the previous plot. e.g.:

plot(0:10, 0:10*3)
plot(identity, add=T, xlim=c(0,10))
plot(function (x) { sin(x)*10 }, add=T, xlim=c(0,10))
par(new = TRUE)
plot(c(2, 3, 4), c(20,10,15), pch="A",
     axes = FALSE, ## don't redraw the axes 
     xlab = '', ylab = '', ## no fresh axis labels
     xlim = c(0,10), ylim = c(0,30)) ## keep the same limits as before

As Ben Bolker mentions, methods('plot') will show you what methods can be called when running plot() - the different methods have different arguments, which are listed when you call args(plot.foo) or in the help page ?plot.foo