title: Don't plot graph on null value on ngx-charts-line-chart
body: <p>I, am using ngx-charts-line-chart to plot the graph. Getting an error on Null value  and the graph is ploted as 0 on the null value.</p>
  
  <pre><code>&lt;ngx-charts-line-chart [results]="progressPerformance"
                             [scheme]="colorScheme"
                             [xAxis]="true"
                             [yAxis]="true"
                             [legend]="false"
                             [showGridLines]="false"
                             [showYAxisLabel]="true"
                             [yAxisLabel]="yAxisLabel"
                             [curve]="curve"&gt;
      &lt;/ngx-charts-line-chart&gt;
  </code></pre>
  
  <p><strong>Errors</strong></p>
  
  <pre><code>ERROR TypeError: Cannot read property 'toLocaleString' of null
      at CircleSeriesComponent../src/common/circle-series.component.ts.CircleSeriesComponent.getTooltipText (index.js:8258)
      at Object.eval [as updateDirectives] (CircleSeriesComponent.html:37)
      at Object.debugUpdateDirectives [as updateDirectives] (core.js:11922)
      at checkAndUpdateView (core.js:11315)
      at callViewAction (core.js:11556)
      at execEmbeddedViewsAction (core.js:11519)
      at checkAndUpdateView (core.js:11316)
      at callViewAction (core.js:11556)
      at execComponentViewsAction (core.js:11498)
      at checkAndUpdateView (core.js:11321)
  </code></pre>
  
  <p>I don't want to plot the line on null value. How can I achieved this.</p>
  
  <p>Here is the screen shot of my graph plot when the API value in null as well.</p>
  
  <p><a href="https://i.stack.imgur.com/Y1sbS.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/Y1sbS.png" alt="screen shot"></a></p>
  
  <p>As per the graph the 18 Feb has value null. The graph should not be plot. on that day and when i hover on that day getting the above errors.</p>
  
  <p>API response<a href="https://i.stack.imgur.com/QDuvT.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/QDuvT.png" alt="api data"></a></p>
  
  <p>Reference issue found on the github - <a href="https://github.com/swimlane/ngx-charts/issues/799" rel="nofollow noreferrer">https://github.com/swimlane/ngx-charts/issues/799</a>. Saying need to add <strong>makeNullPointInvisible with a default value of False</strong> but don't know where to add</p>