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Inside the ranking keywords list you should have a list of all your keywords. If you mouse over a specific keyword on the left listing a little clock icon and a little square graph icon should appear. If you click on the clock icon it should take you to a historic view of that keyword's performance. I have no idea if it backfills to the beginning of your campaign though (e.g. you added a keyword last week, it might not show anything before last week). EDIT-Just tested and it doesn't backfill.