PC plot of all zero scalar is all blue but all zero vector gives nothing and "plot yielded no data"

[markcmiller86]

Describe the bug

1. Create an expression that is a zero vector everywhere.
2. Do a vector plot of it.
3. You get a blank image and a plot yielded no data warning.
4. If you do similar for a PC plot of all zero scalar, you get a plot and no warning.
5. We know the glyphs are all zero length but should we still plot them as vectors with no stems?

It had me confused for a second and I think occasional user would be equally confused.

Not sure its worth the effort and so maybe just document it.

[markcmiller86]

• @biagas observed that we probably don't do anything special to filter out zero vectors and could perhaps wind up in a situation where the default behavior of plotting 400 vectors could wind up selecting randomly 400 zero vectors even though the full dataset contains a lot of non-zeros and wind up telling the user, falsely, the plot yielded no data.
• We believe that the VTK glyphing logic may be removing zero-sized glyhps.
• We could maybe decide, when all glyphs we want to plot zero to convert to point glyph or something.

[markcmiller86]

In looking at vtkGlyph3D, it looks like you can define more than one glyph type to use for the object, indexed by vector magnitude ranges. So, we could define a sphere or box for zero magnitude vectors and a normal vector glyph for everything else. That might fix this behavior.