graph.on('edge:mouseenter', ({ cell }) => {
      if (!cell.hasTool('source-arrowhead'))  {
        cell.addTools('source-arrowhead')
      }
      if (!cell.hasTool('target-arrowhead')) {
        cell.addTools('target-arrowhead')
      }
    })
    graph.on('edge:mouseleave', ({ cell }) => {
      if(cell.hasTool('source-arrowhead')) {
        cell.removeTool('source-arrowhead')
      }
      if(cell.hasTool('target-arrowhead')) {
        cell.removeTool('target-arrowhead')
      }
    })