diff --git a/tox.ini b/tox.ini
index d3a5fb0..b3a8e2a 100644
--- a/tox.ini
+++ b/tox.ini
@@ -8,6 +8,7 @@ deps =
      pyquery
      numpy
 
+# scipy cannot be installed by pip if numpy is not totally installed before
 commands=
   pip install scipy
   py.test [] --cov {envsitepackagesdir}/pygal {envsitepackagesdir}/pygal/test