The X-axis, which is the horizontal axis, on most Excel charts does not use numeric intervals like the vertical Y-axis does. The X-axis contains either strings of text or a date under each set of ...

If you have uneven distribution of data in your Excel chart, here is a better way to change the automatic axis settings.

Graphs in Microsoft Excel typically chart time on the horizontal axis. Time is the independent variable in most relationships and dependent variables appear on the vertical axis.