vba to set line chart series as gradient line with two colours

Hi

I thought I could assist this poster but embarrassingly have fallen at the first hurdle

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what is the VBA code to set a line chart series as a gradient line with two colours? I can use VBA to set gradient fill to the interior of bar and column charts, and even markers on a line chart, but not the line itself.

p.s. I created a new post as

a) its only a sub-set of the poster's problem
b) I didn't want it to appear that his post had been answered; and
c) maybe the charting forum is a better place to post

thanks Andy. The math would have been challenging even if I could figure out the formatting.
it seems a bit odd that such straightforward formatting can't be done with VBA.