Scatter charts use two columns of data for each point. By default, Excel will use the first column for the X coordinate and the second for the Y coordinate. To add additional data for the Y axis ...

How to Add a Second Y-Axis on Excel & Make a Second Series at the End of the Chart. Microsoft Excel charts give you a large number of options for displaying the data in your spreadsheet.

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