Hi Hans,
Thank you very much for the reply.
The graph from windows is a combo of one line and one scatter (with smooth line). But I was able to see and set x values for both series in the same plot. In Excel for Windows, it seems that I am able to define x values for all the graph types.
With your reply and the hints, I am now able to create a small work around to solve the issue:
I first draw scatter plots (as you mentioned, x value only goes with scatter and bubble), where I am allowed to set x values.
Then I set it as combo (excel for Mac also does not offer customised combo option, so I select one of the three options given, and then select the series I want to change the chart type of and change it to my needs).
Since now the box for x values would not disappear after changing the chart type, I can do whatever I want with it.
And this solves my problem perfectly. Thank you a lot for the additional information and the very fast response!
Best regards,
Alex