DOC: improve figures in demo/wp_scalogram.py (#769)

In the second plot, updated Fs to 512;
`ax2.specgram(data, NFFT=64, noverlap=32, Fs=512, cmap=cmap,
             interpolation='bilinear')`

again in the second plot, added aspect='auto', and did corrections to the time (x axis) and y axis (scale) with `extent=[0, 1, 1, values.shape[0]]`

ax3.imshow(values, origin='lower', extent=[0, 1, 1, values.shape[0]], 
           interpolation='nearest', aspect='auto')

Figure 2-subplot 2 in this version is compatible with Figure 1-subplot 2 y-axis labels for the wavelet packet names

demo/wp_scalogram.py

@@ -38,12 +38,13 @@
 # Show spectrogram and wavelet packet coefficients
 fig2 = plt.figure()
 ax2 = fig2.add_subplot(211)
-ax2.specgram(data, NFFT=64, noverlap=32, Fs=2, cmap=cmap,
+ax2.specgram(data, NFFT=64, noverlap=32, Fs=512, cmap=cmap,
              interpolation='bilinear')
 ax2.set_title("Spectrogram of signal")
 ax3 = fig2.add_subplot(212)
-ax3.imshow(values, origin='upper', extent=[-1, 1, -1, 1],
-           interpolation='nearest')
+ax3.imshow(values, origin='lower', extent=[0, 1, 0,  len(values)], 
+           interpolation='nearest', aspect='auto')
+ax3.set_yticks(np.arange(0.5, len(labels) + 0.5), labels)
 ax3.set_title("Wavelet packet coefficients")