Merge pull request #1000 from coldav/colin/fix_docs_artifact

[CI] Fix docs graph generation

Author: Colin Davidson

.github/workflows/docs.yml

@@ -37,7 +37,7 @@ jobs:
       - name: Install apt package
         run: |
           sudo apt-get update
-          sudo apt-get install -y doxygen virtualenv
+          sudo apt-get install -y doxygen virtualenv graphviz
 
       - name: Install prerequisites
         run: python -m pip install -r doc/requirements.txt