If I have understood you correctly, you dont need an onclick event (there is none). You need to set the calculation condition (chart properties | General, to make the chart display, but empty) or the show condition (chart peoperties | Layout, to hide the chart), and the condition expression would be
=GetSelectedCount(date) = 1
In the former option, you can also override the 'calculation condition unfulfilled' error with something more meaningful to your users, such as 'Select a date'. See chart properties | General | Error Messages
Hope that helps
Edit: beaten to the punch!
Logic will get you from a to b. Imagination will take you everywhere. - A Einstein