Visualization of potential isn't correct for negative values of V0 in 1D_potential_barrier.py

[sanketsharma]

I am running the example 1D_potential_barrier.py. In the function potential barrier, if set V0 to a negative value then the visualization of the potential isn't correct. For negative values of V0 the potential is -1 when it should be 0 and -2 when it should be the V0.

I think the issue lies in line 432 of the animate function.