I think that looking at the pixels is a separate issue - the assertLinesDrawn test appears useful, but it was malfunctioning and counting lines as drawn that weren't actually being stroked.

I've fixed CanvasAssertions in https://github.com/klausw-g/dygraphs/commit/80348ed3c9e677fb5ff1ef393056e1e2bea1bbf7 to distinguish stroke from fill. It appears that the existing missing_points test was expecting the wrong inflated values, I fixed them to match the actual number of lines drawn. (Not counting the edges of filled regions which have a different color anyway.)