SFViewer: Scatter Plot for Values

[tknopp]

Implement a scatter plot for the system matrix values where each data point is illustrated in the imaginary plane. SFView by U. Heinen has this feature and it looks like this

This can be implemented with the functions offered by Cairo but it needs to be done in a low-level fashion, i.e. draw every primitive. The coloring is a nice feature but it might be easier to first implement this without colors.