Start in the SEO --> Rank Tracker module of any specific client campaign. Scroll down to the keyword table, then click on the keyword for which you wish to see the ranking history.

You'll then be taken to the keyword history page for that specific keyword. You can view the keyword history as a line graph (pictured below) or a table.

Click the time period selector to view the keyword history for that keyword for a particular period of time.

Click the "Settings" icon to choose the historical metrics that are displayed in the table for this keyword.