Generate correct RRD filenames for nested graphs. Fixes #1224
Merge remote branch 'pull/162' into stable-2.0

Closes: #1224
master/graph: fix gfx error when plugin is too old

When a plugin is not updated, its last update might be older than the
start date of the displayed graph. Then the graph cannot be generated
anymore.

This commit generates the graph. It will be empty and will display the
last update date.

Closes: #1358
Merge remote branch 'origin/stable-2.0' into stable-2.0
Merge branch 'fix-1358' into stable-2.0