Charting a Secondary Y Axis in a Charting Panel 
The Trend Chart module type is selected in the Add/Edit Dashboard Panel dialog box.
In the Add/Edit Dashboard Panel dialog box, click Line Chart as the chart type.
Click Advanced to display the Advanced dialog box.
Select Display Secondary Y Axis.
In the Chart Items section, select Left Y Axis or Right Y Axis on each row containing an item, depending on whether you want to associate this series with the left or right Y-axis.