The setting of the specified axis as a decimal fraction of
               the axis unit vector for the first data point of the array
               index with the corresponding value of
               _array_structure_list.axis_set_id.
               If the index is specified as 'increasing', this will be the
               centre of the pixel with index value 1.  If the index is
               specified as 'decreasing', this will be the centre of the
               pixel with maximum index value.