Simple LineGraph

[Enki]

Hey,
I am pretty new to Qlikview and have tried to display something using some videos and different forum posts.
What i am trying to do is to create a simple linegraph chart , but only displaying the highest values within that linegraph. I tried to use stuff like:     above,below,after,before,peek   functions but still didn't manage to display it.

Can someone help me what function i have to use so that only the 'peek' values are shows in my linegraph?
in the pictures you can see my current settings and displayed graph.
My current expression is: 
If(PD_Roh.Giessen_Ofendruck_bei_Gießbeginn_in_mBar<400 ,null(),PD_Roh.Giessen_Ofendruck_bei_Gießbeginn_in_mBar)

[Enki]

The Expression is very bad see so i have added an extra picture.