The element opacity support is somewhat inconsistent. This is too bad as opacity is useful when rendering very large graphs. There are a number of low hanging fruits

• Preview
  • Add opacity to the custom color picker for Node/Edge/Label.
  • Edge opacity in PDF does not use the "original" color
• Appearance/Ranking
  • The color gradient component doesn't allow for opacity configuration

Limitations

• The "Overview" visualization doesn't support opacity. This should be addressed separately in the new viz engine.
• When alpha == 0 on edges, the edge color on Overview switches to source/target color. This is a bug due to the fact we used alpha = 0 as a flag. So for ranking, one should avoid 0 for now.
• The color chooser on Appearance/Partition and Appearance/Unique doesn't support opacity setting. I've reported this issue to the library author, let's see.
• The Edit panel doesn't yet support opacity setting. Something to follow-up on.

Notes

• The node border opacity, as well as the label outline opacity are inherited from their parent. I don't really see too much of a use-case having the border transparent while the rest not. But it would be feasible if needed.
• There is also a property for edge opacity in Preview. In the case, the edge original color already has an alpha < 1, we will multiply the two alphas.