Werk #8499: Graph designer: handle invalid user input where vertical range is negative

Component	Metrics system
Title	Graph designer: handle invalid user input where vertical range is negative
Date	Apr 29, 2016
Level	Trivial Change
Class	Bug Fix
Compatibility	Compatible - no manual interaction needed
Checkmk versions & editions	1.4.0i1 Checkmk Pro, Checkmk Ultimate MT

This avoids totally breaking the graph in that case.

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