docs: function graphvizualize(data, cb) {
    var cp = exec('dot -Tsvg');
  
    // buffer stdout
    var buf = '';
    cp.stdout.on('data', function (data) {
      buf += data;
    });
    cp.on('error', function () {});
    cp.on('close', function (code) {
      cb(null, code > 0 ? '' : buf);
    });
  
    // set dot to stdin
    cp.stdin.end(data);
  }