Fix confusing code around plot marker size vs stroke width

[emilk]

What

• Part of Expose ui based radii, remove view default radius setting #6540

The existing code was using the same field to mean different things at different times. I have no idea if the original code was buggy, since it didn't document expected behavior.

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[Wumpf]

we should note the difference in diameter vs radius in the migration guide as well then 🤔

[emilk]

Assuming there is a change. The previous MarkerSize did not document if the size referred to the radius, diameter, or something else.