code: def _get_altair_html_(self, chart_obj, slug):
          
          try:
              json_data = chart_obj.to_json(indent=0)
          except Exception as e:
              self.err(e)
          html = '<div id="' + slug + '"></div>\n'
          html += '<script type="text/javascript">'
          html += 'var spec = ' + json_data.replace("\n", "") + ";"
          html += """
          var embed_opt = {"mode": "vega-lite"};
          function showError(altel, error){
              altel.innerHTML = ('<div class="error">'
                              + '<p>JavaScript Error: ' + error.message + '</p>'
                              + "<p>This usually means there's a typo in your chart specification. "
                              + "See the javascript console for the full traceback.</p>"
                              + '</div>');
              throw error;
          };\n"""
          html += "const el_" + slug + " = document.getElementById('" + slug + "');"
          html += "vegaEmbed('#" + slug + "', spec, embed_opt)"
          html += ".catch(error => showError(el_" + slug + ", error));"
          html += '</script>'
          return html
docs: Get html for an Altair chart