code: function(evt, x, y) {
  
          if (!this.can('vertexAdd')) return;
  
          // Store the index at which the new vertex has just been placed.
          // We'll be update the very same vertex position in `pointermove()`.
          var vertexIdx = this.addVertex({ x: x, y: y }, { ui: true });
          this.eventData(evt, {
              action: 'vertex-move',
              vertexIdx: vertexIdx
          });
      }
docs: Drag Start Handlers