title: dataframe.js - Is it possible to sum multiple columns in a grouped dataframe?
body: <p>This question is specific to <code>dataframe.js</code>.</p>
  <p>Here is the test data I am using</p>
  <pre><code>let data = [
      {
          year : 2020,
          v : 0.1,
          cnt_1 : 1,
          cnt_2 : 20
      },
      {
          year : 2020,
          v : 0.1,
          cnt_1 : 3,
          cnt_2 : 20
      },
      {
          year : 2020,
          v : 0.1,
          cnt_1 : 5,
          cnt_2 : 4
      },
      {
          year : 2020,
          v : 0.1,
          cnt_1 : 7,
          cnt_2 : 20
      },
      {
          year : 2020,
          v : 0.2,
          cnt_1 : 9,
          cnt_2 : 20
      },
      {
          year : 2020,
          v : 0.2,
          cnt_1 : 11,
          cnt_2 : 20
      },
      {
          year : 2021,
          v : 0.2,
          cnt_1 : 13,
          cnt_2 : 20
      },
      {
          year : 2020,
          v : 0.1,
          cnt_1 : 15,
          cnt_2 : 20
      },
      {
          year : 2021,
          v : 0.1,
          cnt_1 : 17,
          cnt_2 : 20
      }
  ];
  </code></pre>
  <p>And The result I expected looks like ...</p>
  <pre><code>| year      | v         | cnt_1_sum | cnt_2_sum |
      ------------------------------------
      | 2020      | 0.1       | 31        | 84        |
      | 2020      | 0.2       | 20        | 40        |
      | 2021      | 0.2       | 13        | 20        |
      | 2021      | 0.1       | 17        | 20        |
  </code></pre>
  <p>I could do that with single column like below. But got no idea with multiple columns.(In this case, <code>cnt_1</code> and <code>cnt_2</code>)</p>
  <pre><code>let df = new DataFrame(data);
  let grouped = df.groupBy('year', 'v');
  let cnt1_sum = grouped.aggregate(grpObj =&gt; grpObj.stat.sum('cnt_1')).rename('aggregation', 'cnt_1_sum');
  cnt1_sum.show();
  // and shows below
  | year      | v         | cnt_1_sum |
  ------------------------------------
  | 2020      | 0.1       | 31        |
  | 2020      | 0.2       | 20        |
  | 2021      | 0.2       | 13        |
  | 2021      | 0.1       | 17        |
  </code></pre>
  <p>The only way I know is join 2 dataframes with <code>year</code> and <code>v</code>. But it is so ... inefficient when there are multiple grouped columns.(if got 8 columns then should I have to join 8 dataframes?)</p>
  <p>So here is the question. It there anyway to</p>
  <ul>
  <li>apply <code>stat</code> function to multiple columns ?</li>
  <li>add a column with data ? (<code>withColumn</code> API is not working with plain array)</li>
  </ul>