this is exactly how it should work - see also Template widget - tutorial & examples - make your own widget! - #145 by Foxejoe
The question is, what type does getValue of your item Thermostat_Buero_SetPointTemperature return? Is it a DecimalType?
Very cool graphics by the way 
Also, here (Template widget - tutorial & examples - make your own widget! - #142 by Christoph_B) is another possible solution:
ng-click=“sendCmd(‘EGWZHZThermostat_4_SetTemperature’, 1.0*itemValue(‘EGWZHZThermostat_4_ActualTemperature’) + 0.5)”