Excel automatically distributes these points along a uniform X axis. To add more data to the Y axis in a line chart, add additional numbers in cells below your existing data.

This article explains how to add a secondary axis to a chart in Excel so you can view unlike categories on the same graph. These instructions work in Excel in Microsoft 365, Excel 2021, Excel 2019 ...

If you instead make a chart from multiple rows and multiple columns, Excel will make one dimension the x-axis and make the other into different series.