I'm trying to draw circles with the scatter function. They are supposed to represent trees in the savannah. It is thus important that they are displayed with a proper size, that is, one which represents their actual size on the field. After quite some confusion, I've found out (I think) that the size argument one can specify with the scatter function is given as a disk's surface in pixels square (I think that's what means the "points^2" in the documentation and from my own tests)
What I would like is to give a surface in unit^2, where "unit" is the unit of my data, and which you can read on the plot's axes ticks. For example, each tree has coordinates like x=3500, y=2210. (The unit here is centimeters but we don't really need to know this). Say I want to draw a tree which canopy is 200 cm wide. That makes a disk which radius is 100, or surface 100^2*PI. How can I draw this?