code: function _downHandler(e) {
        if (_settings('mouseEnabled')) {
          _startCameraX = _camera.x;
          _startCameraY = _camera.y;
  
          _lastCameraX = _camera.x;
          _lastCameraY = _camera.y;
  
          _startMouseX = sigma.utils.getX(e);
          _startMouseY = sigma.utils.getY(e);
  
          _hasDragged = false;
          _downStartTime = (new Date()).getTime();
  
          switch (e.which) {
            case 2:
              // Middle mouse button pressed
              // Do nothing.
              break;
            case 3:
              // Right mouse button pressed
              _self.dispatchEvent('rightclick',
                sigma.utils.mouseCoords(e, _startMouseX, _startMouseY));
              break;
            // case 1:
            default:
              // Left mouse button pressed
              _isMouseDown = true;
  
              _self.dispatchEvent('mousedown',
                sigma.utils.mouseCoords(e, _startMouseX, _startMouseY));
          }
        }
      }
docs: The handler listening to the 'down' mouse event. It will start observing
  the mouse position for dragging the graph.
  
  @param {event} e A mouse event.