Supposing you have created a line chart as below screenshot shown. Now you can add a right hand Y axis to the line chart as follows:

1. Right click the chart, and click Select Data from the right-clicking menu. See screenshot:

2. In the Select Data Source dialog box, please click the Add button.

3. In the Edit Series dialog box, please specify the series name and series values exactly same as original series, and then click the OK buttons successively to close the Edit Series dialog box and the Select Data Source dialog box.
Note: In my case, I specify the Series name as Cell B1, and specify the series values as Range B2:B16.

4. Now right click the line in the chart, and select Change Series Chart Type from the right-clicking menu. See screenshot:

5. In the Change Chart Type dialog box, check the second checkbox in the Secondary Axis column, and click the OK button. See screenshot:

6. Now you return to the chart again. Right click the left Y axis, and select Format Axis in the right-clicking menu. See screenshot:

7. In the Format Axis pane, please activate the Text Options, and then check the No Fill option. See screenshot:

Now you will see a right-hand Y axis is added in the line chart. See screenshot:

If you are using Excel 2010, you can add a right hand/side Y axis in a chart as follows:

1. Follow the Step 1-3 introduced in above method to add the second data series.

2. Right click the new line in the chart, and select Format Data Series in the right-clicking menu. See screenshot:

3. In the Format Data Series dialog box, enable Series Options in the left bar, check Secondary Axis option, and close the dialog box. See screenshot:

4. Select the left Y axis in the chart, and then click Home > Font Color > White (or other color same to the chart background). See screenshot:

Now you have added the Y axis on the right hand in the chart. See screenshot:

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