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 parameter add=TRUE
with no problem:
plot(0:10, 0:10*3)
plot(identity, add=TRUE, xlim=c(0,10))
plot(function (x) { sin(x)*10 }, add=TRUE, xlim=c(0,10))
But when I issue
plot(c(2, 3, 4), c(20,10,15), add=TRUE, pch="A")
It doesn't work!! It says that "add" is not a graphical parameter.
Please do not write that I should use points()
instead. I know I can use it.
I want to understand the strange behaviour of R - why does it sometimes work and sometimes not?
This is admittedly annoying and inconsistent, but it's explicable.
edit: the fact that identity
is a built-in object (identity function) eluded me (so the problem is in fact reproducible).
identity
is an object of a class -- function
-- that has a plot
method (plot.function
) with an add
argument, while the default plot
method does not have an add
argument.
In general, when trying to plot object bar
, you should try class(bar)
; if it is of class foo
then try methods(class="foo")
to see that it has a plot method, or methods("plot")
to see that plot.foo
exists. Try ?plot.foo
to see help, or plot.foo
or getAnywhere(plot.foo)
to see the function itself.