Support vtkLegendScaleActor with add_legend_scale

[banesullivan]

Overview

Adds support for vtkLegendScaleActor which is a widget style actor for adding a sense of scale to the scene. This has tons of potential configurations options and I tried to expose what I could and make it similar to add_ruler().

Also cleans up the related add_ruler() method by moving it to the `Renderer`` class and adding color setters to resolve #3888

Details

import pyvista as pv

p = pv.Plotter(notebook=0)
p.add_mesh(pv.Cone())
p.renderer.add_legend_scale()
p.set_background('darkgrey')
p.show()

To do

• Set the text and component colors to match the active theme
• Decide if *_axis_visibility should be on or off by default
• Add example to docstring
• Add test
• Converge with add_ruler to make sure APIs are similar
• Fix any issues with add_ruler - e.g., text color

Known Issues

The label colors are either white or black. This seems to be an upstream bug in VTK where if the color is set at all, it turns the label black.

[codecov]

Codecov Report

Merging #3716 (37f6b54) into main (86d7356) will increase coverage by 0.00%.
The diff coverage is 93.15%.

@@           Coverage Diff           @@
##             main    #3716   +/-   ##
=======================================
  Coverage   94.18%   94.19%           
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  Files          91       91           
  Lines       19586    19631   +45     
=======================================
+ Hits        18448    18492   +44     
- Misses       1138     1139    +1     

[MatthewFlamm]

See #2531 which implements the ability to add the rulers using vtkAxisActor2D. vtkLegendScaleActor uses vtkAxisActor2D under the hood with some defaults, but it also has the ability to add that black and white thingy.

I am pointing this out so the documentation can link from one to the other (See Also section) and that we should try to keep the API consistent (maybe necessitating changes to the code from the prior PR).

[banesullivan]

Ah, I missed that PR and that feature - thank you for linking!

[MatthewFlamm]

Mostly docstring suggestions

[MatthewFlamm]

Why are the defaults not the same as recommended for all of these?

[banesullivan]

The defaults match what's set as default in VTK and make the actors visible in testing. The recommended values make them look quite a bit nicer for PyVista's default window_size or bigger

[MatthewFlamm]

This is a hard choice then. I don't have a strong opinion, so leave it up to you and or other reviewers.

[banesullivan]

Thanks @MatthewFlamm!

[MatthewFlamm]

LGTM