ScatterplotGraph: Group points within dense cells

[janezd]

Issue

Fixes #7064.

Description of changes

@ajdapretnar might get her new year's gift. :)

This doesn't check for actual overlapping of points. It divides the plotted area into 50-pixel grid and plots cells with > 10 points as bigger circles or pie charts (in case there are multiple colors).

The change is mostly constrained to the paint method and I'd prefer to keep it this way. The scatter plot graph is too complex to be poked. :)

Aggregation is disabled if:

• widget shows subset or selection,
• jittering is enabled,
• labels are shown,
• a particular widget opts out by implementing a method allow_aggregation that returns False.

grouping-scatter.mov

grouping-geo.mov

To do:

• opt out where necessary, or implement opt-in (how?)
• tests

Includes

• Code changes
• Tests
• Documentation