Thread Subject: making 'size' variable in scatter correspond to axis units

I am wondering how to interpret the S variable in
scatter(X,Y,S)
The documentation states S is the "area of each marker (specified in points^2)", so how can the area in points^2 be related to the axis units? Here is a concrete example: scatter a bunch of spheres of different radii in Euclidean plane such that the marker (circle) correctly shows the radius, in the same units as the axes.
Thanks,
-naor