diff --git a/images/scipy-notebook/Dockerfile b/images/scipy-notebook/Dockerfile
index 55bf7eb5..6f14a27a 100644
--- a/images/scipy-notebook/Dockerfile
+++ b/images/scipy-notebook/Dockerfile
@@ -74,6 +74,7 @@ RUN git clone https://github.com/PAIR-code/facets && \
     fix-permissions "${CONDA_DIR}" && \
     fix-permissions "/home/${NB_USER}"
 
+# Import matplotlib the first time to build the font cache
 RUN MPLBACKEND=Agg python -c "import matplotlib.pyplot" && \
     fix-permissions "/home/${NB_USER}"