Werk #10939: Fixed non functional JSON and PNG export function in graphs

Component	Metrics system
Title	Fixed non functional JSON and PNG export function in graphs
Date	Mar 20, 2020
Level	Trivial Change
Class	Bug Fix
Compatibility	Compatible - no manual interaction needed
Checkmk versions & editions	2.0.0i1 Checkmk Pro, Checkmk Ultimate MT 1.6.0p11 Checkmk Pro, Checkmk Ultimate MT

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