How to translate point size in cex into units of plot in R

问题

I try to figure out how big the radii of the points in my plot in x and y direction are going to be, with their size being set by cex.

I want to create a plotting function which needs this information. It appears to me as if cex would be this magical parameter and nobody actually knows what exactly it is going to do.

I am creating a plot with the png function. I set a graph width and height. I set par(mar) and par(oma). I plot an empty plot with an xlim and ylim being set, and add an additional xaxs = "i" and yaxs = "i". Then I add points.

So, my question: How can I deduce from my graph width and height (set in pixels or inches?), my mar and oma settings (what is their unit???) and my xlims and ylims what the extent of e.g. a point with cex = 2 is going to be?

Thanks!

来源：https://stackoverflow.com/questions/51200677/how-to-translate-point-size-in-cex-into-units-of-plot-in-r