Check_MK Werk 8544: Fixed broken "default grahs" having negative values

ID: 8544 Title: Fixed broken "default grahs" having negative values Component: Metrics System Level: 1 Class: Bug Fix Version: 1.4.0i1 Graph values which have no value specification and thus are rendered with the default rendering method without unit etc. lead to broken graphs when the values were negative. This fixes a regression introduced with 1.2.8p6.