The current legend builder only adds the min and max value to the scale, so there isn’t a possibility to add a scale with intervals marked along the scale.
I think the approach to implement this would be to try to combine an axis alongside the existing ColorScaleElement (by creating a new AxisElement class, to fit in the table layout mechanism, and then having the LegendBuilder return a GridElement combining the two). I was having a look at the code and the tricky part seems to be setting up the List<TickData> that the axis needs for rendering. For axes in the main chart, the list of tick data items is passed to the rendering engine that then updates the anchor points for the labels…then in a later pass, the text labels are rendered at the anchor points. But in a color scale we’d need to maybe use the layout pass to populate the anchor points.
If I get something that works, I’ll send you some (probably incomplete) code. The rest of December is quite busy for me though, so I might not get too much time to look at it.