code: function (d, chart, series) {
              var returnYClusterGap = 0;
              if (series.y.categoryFields !== null && series.y.categoryFields !== undefined && series.y.categoryFields.length >= 2 && series.clusterBarGap > 0 && !series.y._hasMeasure()) {
                  returnYClusterGap = (d.height * ((chart._heightPixels() / series.y._max) - (dimple._helpers.yGap(chart, series) * 2)) * (series.clusterBarGap > 0.99 ? 0.99 : series.clusterBarGap)) / 2;
              }
              return returnYClusterGap;
          }
docs: Calculate the y gap for clusters within bar type charts