I think you can programatically get that ratio you need by doing the following inside the paint method

dpi = self.getViewWidget().metric(QtGui.QPaintDevice.PaintDeviceMetric.PdmPhysicalDpiX)
pixmap_dpi = self.fragmentAtlas.pixmap.metric(QtGui.QPaintDevice.PaintDeviceMetric.PdmPhysicalDpiX)

scale = dpi / pixmap_dpi

Likely don't want to be calling this during every paint call...

I think you can programatically get that ratio you need by doing the following inside the paint method
dpi = self.getViewWidget().metric(QtGui.QPaintDevice.PaintDeviceMetric.PdmPhysicalDpiX)
pixmap_dpi = self.fragmentAtlas.pixmap.metric(QtGui.QPaintDevice.PaintDeviceMetric.PdmPhysicalDpiX)
scale = dpi / pixmap_dpi

Although we call it "hi-dpi", the quantity that we are interested in is the DPR rather than the DPI.

As a bonus, this PR also fixes the following artifacts that appear for the bottom right plot of ScatterPlot.py example the moment the mouse is moved over it.
(Windows 10 device pixel ratio 1.25, PySide6 6.6.0)

Even without a display with DPR 1.25, it's possible to test this out by setting the environment variable QT_SCALE_FACTOR such that the effective DPR is 1.25. e.g. on my DPR 2.0 display, I set QT_SCALE_FACTOR=0.625

The artifacts are not present in Qt 5.15, Qt 6.2.
The artifacts are present in Qt 6.4 onwards.
(6.3 was not tested)

Thanks @pijyoi for the PR, this LGTM merging!