This ability to conditionally refresh the display of the XY Graph block is not available in Simulink 7.5 (R2010a).
As a workaround, you can write an S-function that achieves this functionality. The S-function for the existing XY Graph block is available for editing at the following location:
where $MATLABROOT is the value returned by executing the following command at the MATLAB command prompt:
As an example, see the attached files. This S-function clears the XY Graph data whenever there is a rising edge at the third input port to this block. The period of the pulse generator is set such that the graph will be cleared after each half-circle is drawn.