title: How transform multiple columns in 2
body: <p>I have a dataframe like this way:</p>
  <div class="s-table-container">
  <table class="s-table">
  <thead>
  <tr>
  <th>ColumnA</th>
  <th>ColumnB</th>
  <th>Group A</th>
  <th>Group B</th>
  <th>Group C</th>
  </tr>
  </thead>
  <tbody>
  <tr>
  <td>2000</td>
  <td>monkey</td>
  <td>12000</td>
  <td>4500</td>
  <td>1500</td>
  </tr>
  <tr>
  <td>2000</td>
  <td>cat</td>
  <td>4000</td>
  <td>8000</td>
  <td>910</td>
  </tr>
  <tr>
  <td>2001</td>
  <td>monkey</td>
  <td>14000</td>
  <td>10000</td>
  <td>800</td>
  </tr>
  <tr>
  <td>2002</td>
  <td>monkey</td>
  <td>18000</td>
  <td>21000</td>
  <td>500</td>
  </tr>
  <tr>
  <td>2003</td>
  <td>monkey</td>
  <td>2100</td>
  <td>9000</td>
  <td>2000</td>
  </tr>
  </tbody>
  </table>
  </div>
  <p>I would like to group 3 columns (Group A, Group B, Group C) in 1 column to get this result:</p>
  <div class="s-table-container">
  <table class="s-table">
  <thead>
  <tr>
  <th>ColumnA</th>
  <th>ColumnB</th>
  <th>Groups</th>
  <th>Number</th>
  </tr>
  </thead>
  <tbody>
  <tr>
  <td>2000</td>
  <td>Monkey</td>
  <td>group A</td>
  <td>12000</td>
  </tr>
  <tr>
  <td>2000</td>
  <td>Cat</td>
  <td>group A</td>
  <td>4000</td>
  </tr>
  <tr>
  <td>2000</td>
  <td>Monkey</td>
  <td>group B</td>
  <td>4500</td>
  </tr>
  <tr>
  <td>2000</td>
  <td>Cat</td>
  <td>group B</td>
  <td>8000</td>
  </tr>
  <tr>
  <td>2000</td>
  <td>Monkey</td>
  <td>group C</td>
  <td>1500</td>
  </tr>
  <tr>
  <td>2000</td>
  <td>Cat</td>
  <td>group C</td>
  <td>910</td>
  </tr>
  <tr>
  <td>2001</td>
  <td>Monkey</td>
  <td>group A</td>
  <td>14000</td>
  </tr>
  <tr>
  <td>2001</td>
  <td>Monkey</td>
  <td>group B</td>
  <td>10000</td>
  </tr>
  <tr>
  <td>2001</td>
  <td>Monkey</td>
  <td>group C</td>
  <td>800</td>
  </tr>
  <tr>
  <td>2002</td>
  <td>Monkey</td>
  <td>group A</td>
  <td>18000</td>
  </tr>
  <tr>
  <td>2002</td>
  <td>Monkey</td>
  <td>group B</td>
  <td>21000</td>
  </tr>
  <tr>
  <td>2002</td>
  <td>Monkey</td>
  <td>group C</td>
  <td>500</td>
  </tr>
  <tr>
  <td>2003</td>
  <td>Monkey</td>
  <td>group A</td>
  <td>2100</td>
  </tr>
  <tr>
  <td>2003</td>
  <td>Monkey</td>
  <td>group B</td>
  <td>9000</td>
  </tr>
  <tr>
  <td>2003</td>
  <td>Monkey</td>
  <td>group C</td>
  <td>2000</td>
  </tr>
  </tbody>
  </table>
  </div>