code: public void setZValues(double[][] zValues, double low, double high) {
      this.zValues = zValues;
      this.lowValue = low;
      this.highValue = high;
    }
docs: Replaces the z-values array. The number of elements should match the number of y-values, with
  each element containing a double array with an equal number of elements that matches the number
  of x-values. Use this method where the minimum and maximum values possible are not contained
  within the dataset.
  
  @param zValues the array to replace the current array with. The number of elements in each
  inner array must be identical.
  @param low the minimum possible value, which may or may not appear in the z-values.
  @param high the maximum possible value, which may or may not appear in the z-values.