title: PHP Spreadsheet HTML won't display chart
body: <p>I am trying to display a chart created in PHP spreadsheet in HTML. It is successful if I save to xlsx.</p>
  <p>I'm using the chart creation code straight from the samples on github.
  <a href="https://github.com/PHPOffice/PhpSpreadsheet/blob/master/samples/templates/chartSpreadsheet.php" rel="nofollow noreferrer">https://github.com/PHPOffice/PhpSpreadsheet/blob/master/samples/templates/chartSpreadsheet.php</a></p>
  <pre><code>$writer = new \PhpOffice\PhpSpreadsheet\Writer\Html_Bootstrap($spreadsheet);
  $writer-&gt;setEmbedImages(true);
  $writer-&gt;writeAllSheets();
  $html_writer-&gt;setIncludeCharts(true);
  echo $html_writer-&gt;generateHTMLAll(); //shows table but no chart
  $spreadsheet-&gt;setActiveSheetIndex(0);
  $writer = new Xlsx($spreadsheet);
  $writer-&gt;setIncludeCharts(true);
  $writer-&gt;save('test.xlsx'); //saves file with chart
  </code></pre>
  <p>Update:
  I have tracked down the problem to line 738 of PhpSpreadsheet/Writer/Html.php
  failing to render. I'm assuming the <code>Settings::getRenderer()</code> is returning null.</p>