scatter plots: size in points vs. size in area

I want to be able to specify the size of points in a

    > scatter plot in data coordinates rather than points^2.
    > The points should change in size at different zoom
    > levels. Is there a built-in way to do this, or a good
    > workaround for now?

You can't do this with the built-in scatter or RegularPolyCollection
(which scatter uses) because these assume a size in points. But you
can roll your own PolyCollection fairly easily

import as cm
import matplotlib.numerix as nx
from matplotlib.collections import PolyCollection
from pylab import figure, show

xs, ys, radii, colors = nx.mlab.rand(4,100)
radii *= 0.1

def poly(x,y,r,numsides):
    theta = 2*nx.pi/numsides*nx.arange(numsides)
    return zip(x + r*nx.cos(theta), y + r*nx.sin(theta))

verts = [poly(x,y,r,6) for x,y,r in zip(xs, ys, radii)]
col = PolyCollection(verts)

fig = figure()
ax = fig.add_subplot(111, xlim=(0,1), ylim=(0,1), aspect=1)