         each group is provided in a specific line, each subgroup in
                    a specific column.
                  </li>
                  <br />
                  <li>
                    The <code>d3.stack()</code> function is used to stack the
                    data: it computes the new position of each subgroup on the Y
                    axis
                  </li>
                  <br />
                  <li>
                    The output of <code>d3.stack()</code> can be used to cre