[R] plot changes usr?

[David Winsemius]

On Sep 28, 2015, at 5:33 PM, Ed Siefker wrote:

> I'm trying to plot() over an existing plot() like this:
> 
>> attach(mtcars)
>> plot(mpg, hp)
>> par(new=TRUE)
>> par("usr")
> [1]   9.46  34.84  40.68 346.32
>> plot(mpg, hp, col="red", axes=FALSE, xlim=par("usr")[1:2], ylim=par("usr")[3:4], xlab="", ylab="")
>> par("usr")
> [1]   8.4448  35.8552  28.4544 358.5456

The default usr ranges are some factor (my hazy memory says 104%) of the range of thex- and y-values unless you specify otherwise. This choice allows round data-points to be displayed at the extremes. Why are you trying to muck with the plot setup? The right way would be to use points().


> 
> For some reason "usr" is changing, and so it's not plotting over the
> existing data in the right place.
> 
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David Winsemius
Alameda, CA, USA