specgram

Hi, I am trying to produce a spectrogram for my data set and am having an issue with the color map. My data is filtered between 0.02 and 1.0Hz, but specgram() produces an image in the range 0 to 10Hz. Also the color map is not set properly. I would like to have it so the colormap ranges from the min and max powers obtained by specgram. Anyone know how to do this? My code is below.

Pxx, freqs, bins, im = plt.specgram(data, NFFT=nfft, Fs=sps, detrend=py.detrend_none, window=py.window_hanning, noverlap=nfft/2, cmap=None, xextent=None, pad_to=None, sides='default', scale_by_freq=None)
plt.ylim(0,1)
plt.colorbar()
plt.show()

thanks,
D