code: function _addIOBubble(node, d) {
    var group,
        inputs = d.get('inputs.length'),
        outputs = d.get('outputs.length');
  
    if(inputs || outputs) {
      group = node.append('g');
      group.attr('class', 'io-bubble');
      group.append('use').attr('xlink:href', '#io-bubble');
      translateIfIE(group.append('text')
          .text(_trimText(`${inputs}/${outputs}`, 3)), 0, 4);
  
      translateIfIE(group, -38, -15);
    }
  }
docs: Add IO(source/sink) bubble to a vertex node
  @param node {SVG DOM element} Vertex node
  @param d {VertexDataNode}