diff --git a/.readthedocs.yaml b/.readthedocs.yaml
index 33c2a61..c8e145a 100644
--- a/.readthedocs.yaml
+++ b/.readthedocs.yaml
@@ -12,6 +12,8 @@ build:
   os: ubuntu-20.04
   tools:
     python: "3"
+  apt_packages:
+    - graphviz
 
 python:
   install: