I don't think you can directly, since the trendline is a single object,
so you cannot format just a segment of it.

You could fake it by getting the coefficients of the trendline (if from
the chart, be sure to display in scientific notation with 14 decimal
places to avoid rounding) then remove the trendline from the chart
(because it will always appear on top of everything else) and manually
replot the trendline from cells as two different series divided at your
desired breakpoint.

Jerry

zibel_boy wrote:

If I have a graph with multiple trendlines, and I want to have a certain
trendline in a different color from a certain point, how do I do it without
changing the color of the entire trendline?