Support configuring circle size in the sample plot

[fedarko]

Something I was thinking about while working on #240. This would be useful for folks with visual impairments, and it also makes distinguishing colors a lot easier -- see for example the following two plots where all that's different is the circle size:

Default (size is 30)

Big (size is 250)

I don't think this should be too bad to implement -- should be able to just adapt the code for the bar width slider to this.