Ability to style the shapes of particular data points in the chart

[cjolif]

This might be implemented by giving the ability to get a hand on GFX shapes of a chart (rectangles for bar etc...) in order to style them from the outside without relying on event system of the chart.

This is useful for example to change the color of a data point when selecting the corresponding data item in a map.

[cjolif]

Actually an alternate and maybe better solution would be to not provide a handle on those shape but provides the API that allows them to be dynamically styled

[cjolif]

See #11150 as well.

[cjolif]

In [27978]:

refs #14359. Implement styling functions on plots + fix some plots styling issues. !strict.

[cjolif]

Finally implemented through a styling function. #11150 will be used to implement access to gfx shapes later if needed.