Add support for gradients on edges and nodes #2091

Issue type

Feature request

Description of new feature

Add support for gradients on nodes and edges with the following properties:

Background:

• background-fill : The filling style of the node's body; may be solid (default), linear-gradient, or radial-gradient.

Gradient:

• background-gradient-direction : In case background-fill is linear-gradient, defines the direction of the gradient.
  • may be to-bottom (default), to-top, to-left, to-right, to-bottom-right, to-bottom-left, to-top-right, to-top-left..
• background-gradient-stops-colors : The colours of the gradient stops (i.e. cyan magenta yellow).
• background-gradient-stops-positions : The positions of the gradient stops (i.e. 0% 50% 100%). If not specified (or invalid), the stops will divide equally.

Edge line:

• line-fill : The filling style of the edge's line; may be solid (default), linear-gradient, or radial-gradient.

Edge gradient

• line-gradient-stops-colors : The colours of the gradient stops.
• line-gradient-stops-positions : The positions of the gradient stops (i.e. 0% 50% 100%). If not specified (or invalid), the stops will divide equally.

Motivation for new feature

This allows for programmatic gradients on nodes and edges. This would be possible without native support for nodes (using SVG background images), but it would not be possible for edges without native support. Having native support makes the gradients easier to make for nodes, even if native support is not absolutely necessary.