This has probably been asked alrady but could not find when searching.

I have added other indicators onto the same plot area.  Now, when I click on a trendline and add an alert, it defaults to the indicator formula I wrote as the value to monitor...when I really want to alert on price.  

How do I change it so my alerts are for price and not some other indicator?

It is actually the first time I have heard of this. Thank you for reporting the issue.

Trendline Alerts seem to be defaulting to the indicator in the scale that is furthest to the right. It is not ideal, but I can suggest a workaround. If you add another Price History indicator to the same pane and scale as the last indicator added, you should be able to get the trendline alerts based on price.

Adding and Moving Indicators