code: function replaceUrlWithGraph(node, vFile) {
    const { title, url, position } = node;
    const { destinationDir } = vFile.data;
  
    // If the node isn't mermaid, ignore it.
    if (!isMermaid(title)) {
      return node;
    }
  
    try {
      // eslint-disable-next-line no-param-reassign
      node.url = renderFromFile(`${vFile.dirname}/${url}`, destinationDir);
  
      vFile.info('mermaid link replaced with link to graph', position, PLUGIN_NAME);
    } catch (error) {
      vFile.message(error, position, PLUGIN_NAME);
    }
  
    return node;
  }
docs: Given a node which contains a `url` property (eg. Link or Image), follow
  the link, generate a graph and then replace the link with the link to the
  generated graph. Checks to ensure node has a title of `mermaid:` before doing.
  
  @param   {object}  node
  @param   {vFile}   vFile
  @return {object}