To view how your keywords position evolve over time, go to your Search Keyword reports. On a particular Keyword, hover on the keyword and click the Row Evolution icon. You will then see a graph showing the evolution of your key metrics (Keyword position, Clicks, Impressions and Clickthrough) over the last few days or weeks or months. You can even compare multiple keywords (for example, the keyword position or number of clicks) at the same time.

Previous FAQ: Where are my search keywords displayed?