about negative polar plots not defined in python not even matlab

[yadin]

high i think everybody has seen dB radiation pattern plots...
well those are done with negative polar plots ...the values of the
magnitude in the axes are
NEGATIVE values...
but i can find a polar plot command that those negative plots not in
python not even in matlab

the polar plots represents the value of the power radiated in dB's and
the dB's go from -40dB to 0dB
as the angle theta changes from 0 to 2*pi rads
the polar plot in python goes with positive values
how can i solve this problem
rough example
example:
power = arange(-40,0,-10)
theta = arange(0, 2pi,pi/12)
polar(power,theta)
title.....?
how can i show the step on the polar plot  plot(-40, -30,-20,-10,0)