code: function(edge, source, target, settings) {
        var color = edge.color,
            prefix = settings('prefix') || '',
            edgeColor = settings('edgeColor'),
            defaultNodeColor = settings('defaultNodeColor'),
            defaultEdgeColor = settings('defaultEdgeColor');
  
        if (!color)
          switch (edgeColor) {
            case 'source':
              color = source.color || defaultNodeColor;
              break;
            case 'target':
              color = target.color || defaultNodeColor;
              break;
            default:
              color = defaultEdgeColor;
              break;
          }
  
        var line = document.createElementNS(settings('xmlns'), 'line');
  
        // Attributes
        line.setAttributeNS(null, 'data-edge-id', edge.id);
        line.setAttributeNS(null, 'class', settings('classPrefix') + '-edge');
        line.setAttributeNS(null, 'stroke', color);
  
        return line;
      }
docs: SVG Element creation.
  
  @param  {object}                   edge       The edge object.
  @param  {object}                   source     The source node object.
  @param  {object}                   target     The target node object.
  @param  {configurable}             settings   The settings function.