Legend Only Shows Used Axis on Dynamic Chart (X, Y and Y2 axis)

I have a dynamic chart that allows manipulation of the X, Y and Secondary-Y (Y2) axis (controlled from a drop-down list). However, there will be times that only the X and Y axis will be used, as no data will be selected for Y2. How can I force the legend to not show a line for Y2, dynamically? Please reference the attached file.
Thanks in advanced for your help!