tds/src/main/java/thredds/server/ncss/controller/NcssGridController.java
protected boolean checkVarsHaveSameVertAxis(CoverageCollection gcd, NcssGridParamsBean params) {
    String zaxisName = null;
    for (String gridName : params.getVar()) {
      Coverage grid = gcd.findCoverage(gridName);
      CoverageCoordAxis zaxis = grid.getCoordSys().getZAxis();
      if (zaxis != null) {
        if (zaxisName == null)
          zaxisName = zaxis.getName();
        else if (!zaxisName.equals(zaxis.getName()))
          return false;
      }
    }
    return true;
  }
protected boolean checkVarsHaveSameVertAxis(CoverageCollection gcd, NcssGridParamsBean params) {
    String zaxisName = null;
    for (String gridName : params.getVar()) {
      Coverage grid = gcd.findCoverage(gridName);
      CoverageCoordAxis zaxis = grid.getCoordSys().getZAxis();
      if (zaxis != null) {
        if (zaxisName == null)
          zaxisName = zaxis.getName();
        else if (!zaxisName.equals(zaxis.getName()))
          return false;
      }
    }
    return true;
  }
Returns true if all the variables have the same vertical axis (if they have an axis).
Could be broadened to allow all with same coordinate unites? coordinate value??