code: function style(edges) {
  
      var marker = vars.edges.arrows.value;
  
      edges
        .attr("opacity", function(d){
          return o_type === "number" ? o :
                 o_type === "function" ? o(d, vars) :
                 vars.edges.opacity.scale.value(d[o]);
        })
        .style("stroke-width",function(e){
          return vars.edges.scale(e[vars.edges.size.value]);
        })
        .style("stroke",vars.edges.color)
        .attr("marker-start",function(e){
  
          var direction = vars.edges.arrows.direction.value;
  
          if ("bucket" in e.d3plus) {
            var d = "_"+e.d3plus.bucket;
          }
          else {
            var d = "";
          }
  
          return direction == "source" && marker
               ? "url(#d3plus_edge_marker_default"+d+")" : "none"
  
        })
        .attr("marker-end",function(e){
  
          var direction = vars.edges.arrows.direction.value
  
          if ("bucket" in e.d3plus) {
            var d = "_"+e.d3plus.bucket
          }
          else {
            var d = ""
          }
  
          return direction == "target" && marker
               ? "url(#d3plus_edge_marker_default"+d+")" : "none"
  
        })
        .attr("vector-effect","non-scaling-stroke")
        .attr("pointer-events","none")
    }
docs: ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Styling of Lines ----------------------------------------------------------------------------