gistfile1.py

#some random data
my_data = np.random.random(2048)

#### Your Code
Fs = 10E6      # Sampling Rate
w_length= 256      # window length
nFFT=2 * w_length
n_overlap=np.fix(w_length/2)
p_to = 8 *w_length

Pxx, freqs, bins= mlab.specgram(my_data,NFFT=nFFT,Fs=Fs,
                                detrend=mlab.detrend_linear,
                                noverlap=n_overlap,
                                pad_to=p_to,scale_by_freq=True)

cp = np.copy(Pxx) #copying to show the trick with indexes

#find a maximum frequency index
maxfreq = 1E5 #replace by your maximum freq
if maxfreq:
    lastfreq = freqs.searchsorted(maxfreq)
    if lastfreq > len(freqs):
        lastfreq = len(freqs)-1

Pxx = np.flipud(Pxx) #flipping image in the y-axis

interp='nearest'
seismic = plt.get_cmap('seismic')
cmap=seismic
fig = plt.figure()
ax1=fig.add_subplot(211)
extent = 0,4,freqs[0],freqs[lastfreq] # new extent
#plot reduced range
img1=ax1.imshow(Pxx[-lastfreq:], interpolation=interp, aspect='auto',
                extent=extent ,cmap=cmap)
fig.colorbar(img1)

#this two images are the same
ax2=fig.add_subplot(212)
img2=ax2.imshow(np.flipud(cp[:lastfreq]), interpolation=interp, aspect='auto',
                extent=extent ,cmap=cmap)
ax1.set_autoscaley_on(False)
fig.colorbar(img2)
plt.show()