title: ChartJS incorrect plot when plotting multiple line charts in one graph
body: <p>I am trying to plot multiple line charts on a graph and I have a toggle to disable / enabled some of the plots. I am destroying / updating whenever required but when I see a particular plot separately, I can see a different plot but when its a mixed graph, the line shows a different plot.</p>
  
  <p>Example fiddle : <a href="https://jsfiddle.net/njmr15w9/" rel="nofollow noreferrer">https://jsfiddle.net/njmr15w9/</a> ( You can try to comment first two plots from the dataset array and you can see how the green plot changes and shows incorrect points, which should not be the case )</p>
  
  <pre><code>    labels: [1, 2, 3, 4, 5,6,7,8],
      datasets: [
        {
            label: "Set 1",
            data: [10, 20, null, 40, 30,null,20,40],
            borderColor: "#F00",
            fill: false,
            steppedLine: false,
            tension: 0,
            fillBetweenSet: 1,
            fillBetweenColor: "rgba(255,0,0, 0.2)"
        },
        {
            label: "Set 2",
            data: [60, 40, 10, 50, 60,null,50,20],
            borderColor: "#00F",
            fill: false,
            steppedLine: false,
            tension: 0.5
        },
        {
            label: "Set 2",
            data: [40, 50, 30, 30, 20,null,60,40],
            borderColor: "#0D0",
            fill: false,
            steppedLine: false,
            tension: 0,
            fillBetweenSet: 1,
            fillBetweenColor: "rgba(5,5,255, 0.2)"
        }
      ]
  };
  
  var chartOptions = {
      responsive: true,
      title: {
          display: true,
          text: 'Demo Fill between lines'
      }
  };
  
  var chartDemo = new Chart($('#demo').get(0), {
      type: 'line',
      data: chartData,
      options: chartOptions
  });
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/hXsum.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/hXsum.png" alt="enter image description here"></a></p>
  
  <p><a href="https://i.stack.imgur.com/YV5zO.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/YV5zO.png" alt="enter image description here"></a></p>