You can create a calculation condition for your chart (on general tab of chart properties), something like
=Count(Distinct Function) <= 10
You can change the error message for the calculation condition unfullfilled also on general tab of chart properties to something like
='Please select less than 11 Function values'
If you create a list box for Function with style Windows checkboxes (on presentation tab), you can select / deselect single values. Note that no selections will still be considered 'all values'.