Thank you very much desantossierra!,

this helps really a lot!

for my understanding i have to map all the x-values, right?

that means for example:
value1: y = 5, x=08:00 clock,
value2: y = 10, x = 08:25...
value3: y= 3, x = 9:45...

so the first value for x is 0800, -> x = 1 ( if 8:00 is the startvalue for the x-axis.)
2nd value x 08:25 -> x = 1,41 - for correct display ?
3rd value x 9:45, -> x = 2,75 - for correct display ?

TIA
Faramia