I updated the Issue due to new things i found out. First off all if you click "Publish combined analysis results" within a job configuration and click on advanced the link "You can define the default values for the trend graph in a separate view." is missing. If you save and reenter the configuration however it is available. If the combined analysis is activated and i configure it so that only one accumulated graph should be shown, nothing happens except the other trend graphs disapear (which is correct). If i trigger the build after that still no graph available. However if the job fails one time and runs properly after the trend graph appears. Maybe there is somekind of cache. I will do more investigation and update this issue. But this is what ive come with so far. Its a Freestyle project btw.
Edit: Another thing i've noticed: If you triggered the job and the graph does not appear, and enter the configuration and change something and save it and retrigger the job the graph will appear.