Animate a custom chart when data changes?

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Has anyone successfully managed to mimic the lerp  ( linear interpolation ) of values for the  chart drawing animation  that Qlik has on the default charts, when data changes?

I have a simple custom extension which I'd like to animate when data changes  - has anyone done this, and how would you persist values outside of the paint() function?

See below, since I'm just drawing the chart from data when paint is invoked, it's binary, whereas the Qlik gauge,  animates both the numeric value  and the chart representation.