NUMBERS: How to add second y-axis to a chart

• Create your columns of data (labels for x-axis on the left, labels for both y-axis at the top of each column of data)
• select all the data in your table (including the row/column of labels)
• from the INSERT menu select CHART > 2D - 2 axis
• 
  

If you've created your table of data and labelled it correctly you will now have a 2D chart with your first group of data shown as a line plot and the left y-axis scaled for this data. Your second group of data will (by default) be a bar chart and the right y-axis will be scaled for this set of data.