in src/matrixDataviewHtmlFormatter.ts [30:37]
public static formatDataViewMatrix(matrix: powerbi.DataViewMatrix): string {
let htmlString = "<div class='datagrid'><table>";
let levelToColumnNodesMap: any[][] = [];
MatrixDataviewHtmlFormatter.countColumnNodeLeaves(matrix.columns.root, levelToColumnNodesMap);
htmlString += MatrixDataviewHtmlFormatter.formatColumnNodes(matrix.columns.root, levelToColumnNodesMap);
htmlString += MatrixDataviewHtmlFormatter.formatRowNodes(matrix.rows.root);
return htmlString += "</table></div>";
}