make sure pychart exists

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@6032 20353a03-c40f-0410-a6d1-a30d3c3de9de

product/ERP5Form/ZPyChart.py

@@ -34,7 +34,10 @@ from Products.Formulator.Field import ZMIField
 from Selection import Selection
 
 from Products.PythonScripts.PythonScript import PythonScript
-import pychart
+try:
+  import pychart
+except ImportError:
+  psychart = None
 
 from zLOG import LOG