code: public void startLink(final VertexContext startVertex) {
          if (startVertex == null) {
              return;
          }
  
          final AbstractLayout<Object, JobGraphLink> graphLayout = _graphContext.getGraphLayout();
          final int x = (int) graphLayout.getX(startVertex.getVertex());
          final int y = (int) graphLayout.getY(startVertex.getVertex());
  
          logger.debug("startLink({})", startVertex);
  
          _startVertex = startVertex;
          _startPoint = new Point(x, y);
  
          transformEdgeShape(_startPoint, _startPoint);
          _graphContext.getVisualizationViewer().addPostRenderPaintable(_edgePaintable);
          transformArrowShape(_startPoint, _startPoint);
          _graphContext.getVisualizationViewer().addPostRenderPaintable(_arrowPaintable);
      }
docs: Called when the drawing of a new link/edge is started
  
  @param startVertex