Use marker size as new dimension in Scatter (See #83)

[dfulachier]

This feature is managed by plotly JS API, and seemed to be already (partially) implemented in plugin code. Feature requested in #83.

[dfulachier]

Add a new "Max" field to be able to control symbols max size. When "Max" field is set:

• Ref field is ignored (it is automatically recomputed)
• The item with max value gets a symbol with this requested Max size
• Other items get a symbol size proportional to the "max" one
• Min is still applied to avoid too small symbols

[gatopeich]

I would like to scale the markers proportional to the X and Y axes rather than pixels, so that a size 1 square on 0,0 would extend from (-0.5, -0.5) to (0.5, 0.5). Would that be possible with your update?