Dimmer switches and analogue voltmeters
Most analog meters actually measure DC current.  In order to have the display change with the the dimmed voltage, you would need to rectify the AC.

Connect the meter through a series resistor to limit the current through the meter to the full scale value at maximum voltage (R = V(max) / I(meter full scale)). Here is a good reference about AC volt meters.

If you happen to already have an analog meter that covers the voltage range that you want, then it has a built in resistor, so you don't need to worry about adding a series resistor.