code: function handleBrushStart() {
              const selection = d3Selection.event.selection;
  
              if (!selection) {
                  return;
              }
  
              dispatcher.call('customBrushStart', this, selection.map(xScale.invert));
          }
docs: When a brush event starts, we can extract info from the extension
  of the brush.
  
  @return {void}