Your diagram has the correct idea but unfortunately won't work. As you have both outputs from the switch joined at the mono jack then they'll also be permanently linked for both channels of the stereo jack. What you need is to either separate the two signals/outputs whenever a stereo plug is inserted, whcih requires a 'switched' stereo jack. Or, connect the two signals when a mono plug is inserted (which is how Rics do it), which requires a 'switched' mono jack.
Stereo switching requires a standard mono jack and something like the normally closed Switchcraft 14B stereo jack (there are other types):
Attachment 24431
Essentially one output from the switch goes to both the switching lug & ring lug on the stereo jack and then to the tip of the mono jack. The other output from the switch goes to the tip lug on the stereo jack. When there's no plug in the stereo jack the tip shorts with the switching lug and thus connects your two outputs for the mono jack, but when a stereo plug is inserted the two outputs are separated as two channels.
Mono switching requires a standard stereo jack and something like the normally open Switchcraft 13A (I think) mono jack:
Attachment 24430
The switch outputs would go to the tip and ring lugs on the stereo jack and then to the tip and switching lug on the switched mono jack. Whenever a plug is inserted in the mono jack the switching lug gets connected and the two outputs are linked for mono output.