about negative polar plots

[yadin]

hi am doing a polar plot of the radiation pattern of an antenna.
the polar plots represents the value of the power in dB's and the dB
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)