Deltavision: conditionally ignore stage position values of 0

If only two unique positions are present in X and Y, and one of them is
0 in both cases, then we ignore the 0 positions (with the result being
that there is only one position).  See gh-1320.

To test, verify that QA 9514 has only one series, as described in gh-1320.

/cc @carandraug