[FIX] ScatterplotGraph: Use mapToView instead of mapRectFromParent

[janezd]

Issue

Fixes #2789.

Description of changes

• To show selection rectangle, (pyqtgraph's) ViewBox.updateScaleBox is overloaded to use self.mapToView instead of self.childgroup.mapRectFromParent. This may not be equivalent if there are multiple viewboxes in the same view or something similarly complicated, but we don't have this.
• To fix selection, (our) mouseDragEvent is modified in the same way
• An unrelated fix: select_by_rectangle assumed that the top left corner is above and to the left of the bottom right. This was true only if the user dragged in this direction. select_by_rectangle now sorts the coordinates.

The PR doesn't pass the diff coverage threshold, but I won't write any tests because they would involve simulating user interaction with mouse, yet at the end there wouldn't be anything to actually test.

Includes

• Code changes

[codecov]

Codecov Report

Merging #3571 into master will decrease coverage by 0.01%.
The diff coverage is 13.33%.

@@            Coverage Diff             @@
##           master    #3571      +/-   ##
==========================================
- Coverage   83.97%   83.95%   -0.02%     
==========================================
  Files         370      370              
  Lines       66944    67005      +61     
==========================================
+ Hits        56213    56257      +44     
- Misses      10731    10748      +17

[VesnaT]

The selection now works, but zoom does not. The mouseDragEvent should probably be modified for zooming as well.

[janezd]

@VesnaT, done. Please check (assuming CI tests pass).

Interestingly, pyqtgraph calls mapToView when showing the rectangle, which is why it was shown correctly while dragging, but then it calls the broken mapRectFromParent at ev.finish. This is why it sufficed to reimplement only a part of mouseDragEvent.