Werk #8484: Fix exception if zooming out too far in graphs.

Component	Metrics system
Title	Fix exception if zooming out too far in graphs.
Date	Mar 21, 2016
Level	Trivial Change
Class	Bug Fix
Compatibility	Compatible - no manual interaction needed
Checkmk versions & editions	1.4.0i1 Checkmk Pro, Checkmk Ultimate MT

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