I've tried your suggestions and the result is tat the range on the Y-axis has reduced a bit, but not as far as I would like. I've attached an image of my graphs.
The area that is indicated in the top graph with red is the area I would like to fill out. As a comparison: the bottom graph automatically behaves as I would like. I don't understand yet, what the difference is.
I've found the issue: What I overlooked when creating the graph is that there were still a whole bunch of invalid ("NULL") data in my graph. By disableing the NULL-values, the graph is now reacting as expected.
As mentioned before I'm new to QlikSense.......but thanks anyway for your assistance.