Is there a tool that can load a web page and compare the space used (and thus load time) by each file? For example:
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Safari browser's Web Inspector (accessible from the Develop menu) has a tab called Resources. In that you can find the list of resources shown in a graph by download time & file size.
I use this often. Try it.
For simplicity, i like View Dependencies, a Firefox extension. Not graphical, but a nice list, just a right-click away. I find Firebug a bit too heavy for what i need to do, and View Dependencies is light.
Download the Google Page Speed add-on for Firebug.
I think Pingdom Tools is right up your alley, though it doesn't show that nifty pie chart.