code: function (chart, series) {
              var returnYGap = 0;
              if ((series.y.measure === null || series.y.measure === undefined) && series.barGap > 0) {
                  returnYGap = ((chart._heightPixels() / series.y._max) * (series.barGap > 0.99 ? 0.99 : series.barGap)) / 2;
              }
              return returnYGap;
          }
docs: Calculate the y gap for bar type charts