title: Chart.js: "stacked" not working with scatter line charts
body: <p>I'm trying to display stacked lines with the chart.js type 'scatter'. Unfortunately, the option "stacked: true" seems not to work here.</p>
  
  <p>Please check the following example (or this <a href="https://jsfiddle.net/faq9tk1j/4/" rel="nofollow noreferrer">this fiddle</a>).</p>
  
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-js lang-js prettyprint-override"><code>var ctx = $('#myChart');
  
  var chart = new Chart (ctx,{
   type:  'scatter',
     data: {
        datasets: [{
          label: 'Line 1',
          fillColor: 'rgb(200,200,200)',
          strokeColor: 'rgb(0,0,0)',
          data: [{
            x: 2020,
            y: 20000
          },{
            x: 2030,
            y: 30000
          },{
            x: 2040,
            y: 40000
          }]
        },{
          label: 'Linie 2',
          fillColor: 'rgb(200,200,200)',
          strokeColor: 'rgb(0,0,0)',
          data: [{
            x: 2020,
            y: 5000
          },{
            x: 2030,
            y: 10000
          },{
            x: 2040,
            y: 15000
          }]
        }]
      }, 
    options: {
      elements: {
        line: {
          tension: 0.000001
        }
      },
      showLines: true,
      scales: {
        yAxes: [{
          stacked: true
        }]
      },
      legend: {
        position: 'bottom'
      }
    }
  });</code></pre>
  <pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;script src="https://cdnjs.cloudflare.com/ajax/libs/Chart.js/2.7.2/Chart.min.js"&gt;&lt;/script&gt;
  &lt;script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"&gt;&lt;/script&gt;
  &lt;html&gt;
  &lt;head&gt;
  &lt;/head&gt;
  &lt;body&gt;
  &lt;canvas id="myChart"&gt;&lt;/canvas&gt;
  
  &lt;/body&gt;
  &lt;/html&gt;</code></pre>
  </div>
  </div>
  </p>
  
  <p>Chart.js Documentation <a href="http://www.chartjs.org/docs/latest/charts/scatter.html" rel="nofollow noreferrer">says</a> "The scatter chart supports all of the same properties as the line chart." and for line charts, there is a "stacked" option (see <a href="http://www.chartjs.org/docs/latest/charts/line.html?h=stacked" rel="nofollow noreferrer">here</a>). In addition, this <a href="https://github.com/chartjs/Chart.js/pull/2867" rel="nofollow noreferrer">issue</a> reported on GitHub also seems to prove, that there is a "stacked" option for scatter charts.</p>
  
  <p>Any idea what I'm doing wrong?</p>