R inconsistency: why add=T sometimes works and sometimes not in the plot() function?
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.
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. eg:
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
上一篇: 在R中快速读取非常大的表格作为数据框