One of the interesting things that graph shows is the IE market share on weekends vs weekdays. It is noticeably lower on weekends, but only by ~5 percentage points.
People often argue that corporations that require employees to use IE are boosting the browser's market share, but from these numbers, that effect seems small. The weekend market share should be a reasonably decent estimate of browser share without the "corporate effect".
I think the "corporate effect" is mostly on IE6. I've seen that people who have a choice to switch from IE6 usually do, but they switch to IE7, not Firefox.
For example, my company gets a ton of hits from IE; about 50% IE7, 25% IE6, and 10% Firefox (our software is for Windows, so we see almost no mac users). The only one of those that drops considerably over the weekend is IE6 (by about 10%) and IE7 spikes nearly as much (8%).
People often argue that corporations that require employees to use IE are boosting the browser's market share, but from these numbers, that effect seems small. The weekend market share should be a reasonably decent estimate of browser share without the "corporate effect".