Drawing QwtPlotCurve without deleting old data

[Uwe]

What should I do, so that the lines will not be replaced and increased until I stop drawing?

Have a look at QwtPlotDirectPainter.
The realtime example shows how to use it. The oscilloscope is probably much closer to your use case, but the source code requires some time to understand how it works.

How can I draw in the center of y-axis, so that the size of the QwtPlot does not matter.

Not sure if I understand this one.

Uwe