Why can't I set up a trendline on a stacked chart in Excel?

[JTX]

I have a stacked area chart that I am trying to add a trendline to, but Excel
won't allow it. However, MS doesn't really say WHY I can't use this feature
on a stacked chart.

1) Does anyone know how to circumvent this limitation of Excel and still
have a trendline on the chart?

2) Does anyone know why MS doesn't have this feature? Seems kinda crazy to
me.

Thanks.