Controlling the radial scale in polar plots

What's the best way to control the radial scale in a polar plot? For example, if I have data like this:

theta = [ 1, 2, 3, 4, 5 ]
rad = [ 10, 20, 30, 40, 50 ]
polar( theta, rad )

How do I make the radial scale go from 0 to 80? I tried messing with the ylim doing something like this:

gca().set_ylim( ( 80*sqrt(2), 80*sqrt(2) ) )

All that seems to do is to scale the whole plot without changing the axis bounds. If I try to set the grid like this:

grids = [ 0, 20, 40, 60, 80 ]
lbls = [ "0", "20", "40", "60", "80" ]
gca().set_rgrids( grid, lbls, frac=1.05 )

The grid lines draw correctly but the background of the plot doesn't change to fit the new grid lines.

Thanks,
Ted
Ted Drain Jet Propulsion Laboratory ted.drain@...369...