This happens when Graphite render returns a 500 message (server side error).
The best way to troubleshoot is to:
- See the webapp log files located usually at /opt/graphite/storage/log/webapp
- Run the graph using the URL API and that way you can see the full exception message
Most times the problem is due to permission problems on the file. Make sure all users (e.g. user running Apache HTTPD server) who need access are able to access the file.