title: Echarts updating series MarkLine without refresh whole chart
body: <p>I have a huge dataset that needs to be rendered in a scatter plot. I am using Vue.js with vue-charts. The problem I am facing is that when I try to update any series (even adding a new one), the whole chart is refreshed with a huge impact on performance.</p>
  
  <p>Here is a codepen where I reproduce the problem:</p>
  
  <p><a href="https://codepen.io/boosterafk/pen/gOaNazR" rel="nofollow noreferrer">https://codepen.io/boosterafk/pen/gOaNazR</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>let seriesData = [];
  let xAxisData = [];
  let yAxisData = ['0','1'];
  for(let i = 0; i &lt; 1800; i++){
    xAxisData.push(i);
    for(let j = 0; j &lt; 3; j++){
      seriesData.push([i,j, .4])
    }
  }
  
  var container = document.getElementById('main');
  var chart = echarts.init(container);
  chart.setOption({
    xAxis: {
      data: xAxisData,
    },
    yAxis: {
      data: yAxisData,
    },
    series: [{
      type: 'scatter',
      data: seriesData,
      markLine: {
         data: [ { name: 0, 
                 xAxis: 10, 
                 label: { 
                   formatter: 0,
                   position: 0,
                 }, 
                 }, ],
      },
    }],
  });
  
  index =10;
  window.setInterval(function(){
    chart.setOption({
    series: [{
      markLine: {
         data: [ { name: index, 
                 xAxis: index, 
                 label: { 
                   position: index,
                 }, 
                 }, ],
      },
    }],
  });
    index+=10;
  }, 1000);</code></pre>
  <pre class="snippet-code-css lang-css prettyprint-override"><code>#main {
    width: 600px;
    height: 400px;
    border: 1px solid red;
  }</code></pre>
  <pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;script src="https://cdnjs.cloudflare.com/ajax/libs/echarts/4.7.0/echarts.min.js"&gt;&lt;/script&gt;
  
  &lt;div id="main"&gt;&lt;/div&gt;</code></pre>
  </div>
  </div>
  </p>
  
  <p>How to fix this problem? What is wrong?</p>