link = d3.select(canvas).append("g")
.attr("class", "links")
.selectAll("line")
.data(graph_data.links)
.enter().append("line")
.attr("stroke-width", function(d) {
return Math.sqrt(d.value);
})
.attr('stroke', d => d.color)
.attr('stroke-linecap', "round")
.attr("marker-end", "url(#end)");