Have you tried putting the Inverter after the Oscillator? I think that when no keys are pressed the Oscillator outputs a LOW signal (zero), so the Inverter should flip that value around to a HIGH signal (in theory) and light up all the LEDs of the Bargraph.
Although, you should keep in mind that the Bargraph takes input from the bit immediately before - in this case the Oscillator. If you want the Bargraph to show the values produced by the Keyboard, then you should plug the circuit like this:
Power > Keyboard > Bargraph > Oscillator
or Power > Keyboard > Inverter > Bargraph > Oscillator
Just as an extra, the Inverter usually takes digital input but it can also be used as a cheap bitcrusher/distorter on sound data or analog signals. Check this thread for more info Favorite extra modules to use with the Synth Kit