From the "Keywords" tab, click the Columns button, which is just under the graph, to the far right. The button displays as three vertical bars. When you click the button, you'll see a dropdown, choose "Modify columns" from the dropdown.
In the next page, you'll see a list called "Modify columns for keywords." Click the "Performance" section, and tick the box marked "Avg. pos." Scroll to the bottom and click "Apply."