thanks for your hint viridis, i'll try it, looks good so far.
but i have another question: how do i find out the position (on y-axis) and height of one drawn curve (in scale coords)?

heightOfYAxis / numberOfCurves
doesn't work of course, because it doesn't consider the extra scale items at all ... but i don't know the height of a scale item and the height doesn't change when the yLeft-scale changes (e.g. when the whole window/widget is resized) ...

thanks in advance!
sincerely,
julian