We're including excanvas as flot/excanvas.min.js, which according to libraries.txt is licensed under Apache 2.0.

https://github.com/metabrainz/musicbrainz-server/blob/master/root/static/lib/libraries.txt#L59

Apache 2.0 is not compatible with GPLv2, and since we've decided not to upgrade to GPLv3 (which is Apache 2.0 compatible), this code should be removed from our source tree, and a replacement should be found, written, or we decide to loose canvas support in internet explorer.