In GitLab 10.3, we added mermaid support, but as with other GitLabFM features, it doesn't render under https://docs.gitlab.com/ee/user/markdown.html#mermaid. This is how the HTML looks:
<div class="highlight"><pre class="highlight plaintext"><code>graph TD; A-->B; A-->C; B-->D; C-->D; </code></pre></div>
According to their docs, it's relatively easy to enable it (https://mermaidjs.github.io/usage.html), but the tricky part is adding the
mermaid class to codeblocks.
In GitLab, there's a filter that does this job and I think we should also extend rouge if we want to build this.
Maybe adding https://github.com/jch/html-pipeline to Nanoc would help us move forward, there's an open MR (https://gitlab.com/gitlab-com/gitlab-docs/merge_requests/97), but that's how far I got.