code: function buildLayers() {
              layers = transformedData.map((item) => {
                  let ret = {};
  
                  groups.forEach((key) => {
                      ret[key] = item[key];
                  });
  
                  return assign({}, item, ret);
              });
          }
docs: Builds the grouped layers layout
  @return {D3Layout} Layout for drawing the chart
  @private