On/off switch with middle state?

We're using an iOS-style on/off switch to toggle selected items on/off. This switch will also reflect the state of the selected items. If you select items that are on, the switch will show on. Select items that are off, the switch will show off.

The issue comes if you select some items that are on and others that are off. For that, I need a version of the switch in some middle position. (Like a checkbox with a dash through it.)

Has anyone seen a version of that kind of switch that includes a middle position? Do any of you have any recommendations regarding my approach?