Visual Implementation (Node Link View)

Built with blockbuilder.org

<!DOCTYPE html>
<meta charset="utf-8">

.node {
  stroke: #fff;
  stroke-width: 1.5px;
}

.link {
  stroke: #999;
  stroke-opacity: .6;
}

.legend rect {
  fill:white;
  stroke:black;
  opacity:0.8;}

</style>
<body>

<h1>Node Link View - Color Coding with starting letter</h1>

var width = 1060,
    height = 600;
    d3.select(self.frameElement).style("height", "900px");
var color = d3.scale.category10();

var force = d3.layout.force()
    .charge(-120)
    .linkDistance(30)
    .size([width, height]);

var svg = d3.select("body").append("svg")
    .attr("width", width)
    .attr("height", height);

d3.json("USstates.json", function(error, graph) {
  force
      .nodes(graph.nodes)
      .links(graph.links)
      .start();

var link = svg.selectAll(".link")
      .data(graph.links)
    .enter().append("line")
      .attr("class", "link")
      .style("stroke-width", function(d) { return Math.sqrt(d.value); });

node.attr("cx", function(d) { return d.x; })
        .attr("cy", function(d) { return d.y; });
  });
});

</script>

Modified http://d3js.org/d3.v3.min.js to a secure url

https://d3js.org/d3.v3.min.js