In this case restrict your chart to display for only one location ( guessing location is the dimension )
Go to Chart Properties -> General Tab -> Calculation condition
write there getselectioncount(" Location ") = 1
this result in warning message Calculation condition Unfulfilled, if user selects more than 1 Location or no Location.
To customized the message , click on Error Messages button and select the above error message ( comes at last ) and write your message, you can do some condition checks too here.