Run the edge detection twice. That way you get better gradients along sharp edges.
Single run: http://i.imgur.com/kusJDRo.png
Double run: http://i.imgur.com/rHYSzpq.png
To me, at least, the double looks better. Especially in the stems.
Just add the following to FrequencyTable.cs, after line 18 (var edges = ...)
detector = new SobelEdgeDetector();
edges = detector.Apply(edges);