we customized the built-in Measure "HTTP 5xx Response (internal)" (which is based on Metric "Web Requests - Response Status Code (Business Transaction Evaluation/Filter/Splitting Values)") by adding a condition "URI starts with /my/path/".
Let's say "HTTP 5xx Response (internal) - /my/path/" the new measure.
Now we just need to chart the total number of internal Web Requests /my/path/ that obtain a 5xx response, but when trying to add the new series, the measure "HTTP 5xx Response (internal) - /my/path/" is not in the list of measures that can be chosen.
The measure "HTTP 5xx Response (internal)" is available instead.
Why in this case a custom measure can't be charted?
I'm not sure the exact reason that may not be showing - perhaps it's due to that measure being a built in one that is used for something important, I can't really say.
Regardless in a case like this it sounds like you simply modified the existing one and changed changed the name. If you haven't done it this way what would be best is to create brand new measure from a "measure template."
This can be done in the system profile settings or anywhere you pick a measure for a chart - you select the "create measure" option towards the bottom right of the window. Then in the screen that opens up find "HTTP Response Status Code" under the "Web Requests" section. Then in the configuration fill in your path, and set the value for the status code to "starts" : 5 - this will make it the same as the built in measure you were looking at. Set the name and you should be good - I expect there would be no issues charting this measure.
I say give this a shot and report if you see the same issue,
The reason is that those measures within Business Transaction Evaluation/Filter/Splitting category are not explicitly evaluated without being a part of a defined business transaction. Your answer is the correct approach 🙂