and try to visualate the axis in Processing based on a cube.
The axis of what? You are sending temperature data (in a strange way) to Processing.
This means that serialEvent() will be called when a carriage return arrives.
Serial.print("Temperatur betraegt ");
Serial.println(" Grad Celsius");
This code is sending strings and TWO carriage returns to Processing. Why?
Then, you send this stuff. Processing will split the string at the :s. Element 0 will be "x". What, exactly, is that supposed to map to a float as? Element 1 will be something like "123<tab>y:". What is that supposed to look like as a float?
Is your Arduino really connected to COM6?