The image above demonstrates a line chart containing two data series and two y-axes, one for each data series. I changed the color of the y-axes so they match the corresponding color of the data series.

A secondary y-axis is very useful if the data points of the second data series are way off. You won't be able to read the chart if they share the same y-axis, here is an example:

This chart shows two data series, however, only one is readable. Here is how to add a secondary axis:

1. Double-press with left mouse button on one of the two data series in the chart to open the task pane.
   
2. Go to tab "Series Options".
3. Select "Secondary Axis".

Add y-axes lines

1. Double-press with left mouse button on left y-axis values to open the settings pane.
2. Go to tab "Fill & Line"
3. Expand Line settings.
4. Press with left mouse button on "Solid Line"
5. Pick a color.
6. Repeat steps 1 to 5 with the right y-axis.

Add major tick marks

1. Go to tab "Axis options".
2. Expand "Tick Marks" settings.
3. Pick a major type tick mark.
   
4. Repeat step 1 to 3 with the right y-axis.

Change the right y-axis boundary

The data series collide and to make it better looking we can adjust the upper and lower y-axis range.

1. Double-press with left mouse button on the right y-axis to open the settings pane.
2. Expand "Axis Options".
3. Change Maximum to 0.2
4. Close the settings pane.