All you folks who have explained the unexplainable, let me add my two cents. It's a bug, not a feature. If I select xy-scatter chart, then I have full access to the X axis labels which I define, as in the earlier Excel, by selecting a column of labels. Everything works like it should, and the chart data menu includes the X axis.
However, switch to a line chart and the X axis is lost. I am told that switching from scatter to area works also.
Again, it's a but, not a feature. Try a different form of chart, like I did (line to scatter) and it might work for you. Meanwhile, come next week, I'll call MS to explain it to them.