ExtJS is open source. (GPLv3) Since Graphite is Apache 2.0, it looks like they're only able to use it due to some sort of exemption from Sencha.
I don't know of any total replacements for ExtJS but something like Sproutcore may come close.
I don't know of any total replacements for ExtJS but something like Sproutcore may come close.
Perhaps I'll look at Sproutcore now that it's had more time to mature.