Never use a 3rd party library to do your core functionality.
This guy's whole project is going to revolve around DOM and Canvas stuff in Javascript. He needs complete control over that, so relying on somebody else's library for it will kill him.
If you're building a business app and need graphics for reporting, then sure, use a library. Use jQuery even. But if you're building a video game, you're going to need to do this stuff yourself.
The author already mentioned that he uses ExCanvas for IE. I don't think it's a stretch to use a library for what would otherwise be tedious recoding of the same logic in both SVG and VML.
If a library helps you deliver the software faster and better, use it. If it gets in your way, then consider using the layer underneath directly.
But, regardless, my main point was to consider browser-based _vector_ graphics, which are several orders of magnitude faster than canvas for activities like this.
This guy's whole project is going to revolve around DOM and Canvas stuff in Javascript. He needs complete control over that, so relying on somebody else's library for it will kill him.
If you're building a business app and need graphics for reporting, then sure, use a library. Use jQuery even. But if you're building a video game, you're going to need to do this stuff yourself.