From The Document Foundation Wiki
How can I design a chart with two Y axes?
- Select the data (3 columns: 1 for the value of the X axis and 2 for the values of the Y axes).
- Insert the chart using ▸ .
- Choose a chart type of XY.
- Create the chart.
- Open the properties of the curve which is to be the secondary axis by one of these methods:
- a triple-click;
- one click, then when the curve is selected (blue squares), right-click and choose Format data series ;
- one click, then go to ▸ .
- on the Options tab, check Secondary Y axis.