Qwt User's Guide  6.1.3
Spectrogram, Contour Plot
spectrogram-rgb-600x400.png
spectrogram-hue-600x400.png
spectrogram-value-600x400.png
spectrogram-saturation-600x400.png
spectrogram-contour1-600x400.png
spectrogram-contour2-600x400.png