Is the height supposed to mean anything? Why did they start with Safari? It's the first modern web browser? It points to a page that says IE9 is not a modern browser yet shows IE7 and IE8.
I decided to start in 2006 to catch Firefox 2 and Internet Explorer 7, otherwise you don't fully see the gaping hole which represents the Dark Ages of the modern web. (Safari was added later, and 2.0 happened to slot in at the start of 2006.)
Edit: My attempt at it. Not as pretty, but a bit more readable:
Even better, roll it back to 1991!
Edit: and another pony request - how about putting the release date in the tooltips (yyyy-mm-dd) to clarify any confusion about when exactly it was?
The reason it shows IE7 and IE8 is to illustrate that all of the modern browsers are shipping pretty regularly, while IE only has two releases on there.