You can't actually have trigger on something being selected in specific table/chart but since selecting in chart actually is making a selection then you can have calculation conditions based on the selected value(s).
Qlikview
When chart properties a open you can have calculation condition. For example:
= GetSelectedCount("CustomerId") = 1

You can overwrite the default error message by specifying the text in Caclulation condiction unfulfilled
section:

(Just to mention that the message can be an expression as well)
Qlik Sense
The situation with Sense is similar:
- select the chart
- expand
Add-ons
- expand
Data handling
- Add the calculation condition
- once the calc condition is added the
Displayed message
input will appead
- enter some message in it to overwrite the default one
