The Alexa Traffic History Graph allows you to create a traffic history graph for any site. Alexa computes traffic rankings by analyzing the Web usage of millions of Alexa Toolbar users.

What is Traffic Rank?
The traffic rank is based on three months of aggregated historical traffic data from millions of Alexa Toolbar users. It is a combined measure of page views and users (reach). The three-month change is determined by comparing the site's current rank with its rank from three months ago.

What is Reach?
Reach measures the number of users. Reach is typically expressed as the percentage of all Internet users who visit a given site. Alexa expresses reach as the number of users per million. The three-month changes are determined by comparing a site's current reach with its values from three month ago.