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 Have Three Secondary Axes on Excel 2007. Excel 2007's secondary axis feature enables you to view two different data series with two distinct scales, all on the same chart. Use this feature ...

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