You can use a "Mux" Block in Simulink with 2 inputs i.e, input 1 would be the output and input 2 would be your reference voltage. Connect the output of the "Mux" Block to the "Scope" block. Now you can see both the signals on the same axis in the "Scope" block. You can edit the color, thickness of the signals in the "Scope" block.
The following link may help for editing the signal view in the "Scope" block.
