Andy,
I can see both series in the list. As far as I can tell, they're both
just regular lines, not stacked. One may have markers, but still just a
regular line. I have no issue adding a trendline if both axis values
are whole numbers. It's the ones with the percentage on the secondary
axis that I am having issues with. Can you tell me what version of
Excel you're using? Maybe I've missed an update somewhere along the
way, but I'm not the only one having this issue and my version doesn't
seem to be the only one either.

I'm so confused! Thanks for your time in trying to help!
Rita



Andy Pope wrote:
Hi,

Which sub-type of line are you using? Stacked lines can not have
Trendlines.

I created a line chart with 2 data series, number and percents. Placing
the percents onto the secondary axis.
When I select either series and right click I can see the Add Trendline
option.

The Trendline dialog, Type tab, allows me to pick which series to base
the trendline on.
Can you see both series in the list, Based on Series: ?

Cheers
Andy