It would always help to attach a small example program so we can see what you are doing. I am not familiar with the "picture.image.front" property.
Are you possibly talking about the "plot images.front" property instead?
I would think that you need to recalculate the line after each zoom change. What kind of graph is it (waveform, xy).
Of course there are always alternative, for example you could generate the line as a second plot.