example/text-links/index.html

<head>
  <style> body { margin: 0; } </style>

  <script src="//unpkg.com/force-graph"></script>
  <!--<script src="../../dist/force-graph.js"></script>-->
</head>

<body>
  <div id="graph"></div>

  <script>
    fetch('../datasets/miserables.json').then(res => res.json()).then(data => {
      const Graph = new ForceGraph(document.getElementById('graph'))
        .graphData(data)
        .nodeId('id')
        .nodeLabel('id')
        .nodeAutoColorBy('group')
        .linkCanvasObjectMode(() => 'after')
        .linkCanvasObject((link, ctx) => {
          const MAX_FONT_SIZE = 4;
          const LABEL_NODE_MARGIN = Graph.nodeRelSize() * 1.5;

          const start = link.source;
          const end = link.target;

          // ignore unbound links
          if (typeof start !== 'object' || typeof end !== 'object') return;

          // calculate label positioning
          const textPos = Object.assign(...['x', 'y'].map(c => ({
            [c]: start[c] + (end[c] - start[c]) / 2 // calc middle point
          })));

          const relLink = { x: end.x - start.x, y: end.y - start.y };

          const maxTextLength = Math.sqrt(Math.pow(relLink.x, 2) + Math.pow(relLink.y, 2)) - LABEL_NODE_MARGIN * 2;

          let textAngle = Math.atan2(relLink.y, relLink.x);
          // maintain label vertical orientation for legibility
          if (textAngle > Math.PI / 2) textAngle = -(Math.PI - textAngle);
          if (textAngle < -Math.PI / 2) textAngle = -(-Math.PI - textAngle);

          const label = `${link.source.id} > ${link.target.id}`;

          // estimate fontSize to fit in link length
          ctx.font = '1px Sans-Serif';
          const fontSize = Math.min(MAX_FONT_SIZE, maxTextLength / ctx.measureText(label).width);
          ctx.font = `${fontSize}px Sans-Serif`;
          const textWidth = ctx.measureText(label).width;
          const bckgDimensions = [textWidth, fontSize].map(n => n + fontSize * 0.2); // some padding

          // draw text label (with background rect)
          ctx.save();
          ctx.translate(textPos.x, textPos.y);
          ctx.rotate(textAngle);

          ctx.fillStyle = 'rgba(255, 255, 255, 0.8)';
          ctx.fillRect(- bckgDimensions[0] / 2, - bckgDimensions[1] / 2, ...bckgDimensions);

          ctx.textAlign = 'center';
          ctx.textBaseline = 'middle';
          ctx.fillStyle = 'darkgrey';
          ctx.fillText(label, 0, 0);
          ctx.restore();
        });
    });
  </script>
</body>