what would the resultant potential graph look like due to the two plates.
Assuming the plates are equal in size, and are arranged symmetrically, and the plates are very large compared to the gap between them and we take a line not too near the edges of the plates, then the graph will be linear.
Is the electric potential at the mid point between two equally oppositely charged plates zero
Still assuming symmetry between the plates, and if the plates are the only two objects in the universe, and potential is defined to be zero "at infinity", then yes.
If the universe is more complex then possibly not. For example, there could be a third, charged, object nearby.
If the zero of potential is defined as other than "at infinity", then possibly not. For example, we could arbitrarily choose one of the plates to have a potential of zero.