code: function (d, chart, series) {
              var returnR = 0,
                  scaleFactor = 1;
              if (series.z === null || series.z === undefined) {
                  returnR = (!series.radius || series.radius === "auto" ? 5 : series.radius);
              } else {
                  if (series.radius && series.radius !== "auto" && series.radius > 1) {
                      scaleFactor = series.radius / series.z._scale(series.z._max);
                  }
                  if (series.z._hasMeasure()) {
                      returnR = series.z._scale(d.r) * scaleFactor;
                  } else {
                      returnR = series.z._scale(chart._heightPixels() / 100) * scaleFactor;
                  }
              }
              return returnR;
          }
docs: Calculate the radius