切换线条时，将箭头的颜色更改为与D3中的线条相同

var marker = svg.append("defs")
  .append("marker")
  .attr("id", "arrow")
  .attr("viewBox", "0 -5 10 10")
  .attr("refX", 10)
  .attr("refY", 0)
  .attr("markerWidth", 6)
  .attr("markerHeight", 6)
  .attr("orient", "auto");

marker.append("path")
  .attr("d", "M0,-5L10,0L0,5")
  .attr("fill", "currentColor");

var line = svg.append("line")
  .attr("x1", x1)
  .attr("y1", y1)
  .attr("x2", x2)
  .attr("y2", y2)
  .attr("stroke", "blue")
  .attr("marker-end", "url(#arrow)");