First of all, I have to say I love Graphviz! :)
GitLab is having issues... See
https://twitter.com/gitlabstatus I just tried to kick off a pipeline and it's not starting either. (It's not even showing up in the list of pipelines even though I did kick it off by clicking "Retry".)
This happens sometimes, it's a public service and load is high right now after releasing GitLab 10.0 on the 22nd, GitLab is still sorting things out.
You can bring up your own GitLab Runners, it's more fast/reliable than using the public ones.
Let us know if you need any help with that.
Thanks!!
Aleksey