after looking around for a plotting library I found Matplotlib and I tried to create a gray scale image in a wxpython application. Looks good!
Now I have to find a line in the image with mainly vertical orientation. To do this a crosshair cursor would be fine. In Pylab I found a SpanSelector which also looks promising.
Unfortunately I failed in adding a crosshair cursor or a SpanSelector into my wxpython application figure.
from matplotlib.widgets import Cursor
If this is not enough, I have once written a Crosshair very similar to SpanSelector - but I don't have it at hand at the moment.