Excel allows you to edit the titles for all of your chart elements, including the horizontal data text. Axis titles communicate the meaning of your chart. Add axis labels to your horizontal axis ...

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.

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