If you are charting more than two series, you can:
chart the series against one of the left/right axes. This would be appropriate if the series contain data within a similar range of values
chart the series against an axis specific for that series. You would do this by checking Use Addition Axis. By default, the axis will appear either to the left of the left axis. If Use Right Axis is also checked, the additional axis will be placed right of the right axis.
You can adjust the position and other properties of the Additional Axes on the Axes tab.