Werk #8843: Fix adding single service metrics to custom graphs

Component	Metrics system
Title	Fix adding single service metrics to custom graphs
Date	Aug 1, 2019
Level	Trivial Change
Class	Bug Fix
Compatibility	Compatible - no manual interaction needed
Checkmk versions & editions	2.0.0i1 Checkmk Enterprise (CEE), Checkmk MSP (CME) 1.6.0b5 Checkmk Enterprise (CEE), Checkmk MSP (CME) 1.5.0p23 Checkmk Enterprise (CEE), Checkmk MSP (CME)

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