CLIMATE-466 - Update plotter for better colorbar

- Update colorbar range to ensure that 0 is centered when there's a
  0 value. Previously when the value range crossed 0 it was
  possible to end up with a range where 0 was pushed very far to one
  side. It's more convenient to analyze a graph when the 0 value is
  centered in the color spectrum.
Resolve CLIMATE-466. Merge PR #67.