QT/C++ graphics scaling

I am using QT/C++ stuff and I need to come up with something like this:

I'm creating a QT container with regions that contains 2D graphics.
these graphics correspond to coordinates such as (time,value) and can be magnified with a zoom method.
the problem is the zoom method has some kind of threshold, so if the current zoom level if less than the threshold we will be using (time1,value1) to draw a certain graphic else it will be drawn using different coordinates, (time2,value2) for example.
Is there a way to achieve this?
Thanks !
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