title: Extract unique values of a key in a object - Javascript/d3 legend
body: <p>Consider the following simplified csv file</p>
  <pre><code>x,y,names
  1,2,group1
  3,2,group2
  4,3,group1
  7,8,group3
  3,5,group2
  </code></pre>
  <p>which I am reading in with d3.csv and afterwards apply a some function on it</p>
  <pre><code>d3.csv('file_name.csv').then(data =&gt; {
      render(dataset)
  });
  </code></pre>
  <p>Could someone explain to me how I could extract the unique strings in the category <strong>names</strong> and store them in a list</p>
  <p><code>---&gt; iam_a_list = [group1, group2, group3]</code></p>
  <p>The elements in this list will later be used as text for a legend in a plot.</p>