Back in February, I hacked together this little site to show the spread between Bitcoin priced in Tether and Bitcoin priced in USD:

http://www.untether.space/

Until today, it's never gone much beyond 1%; as I write this, it's at 8.4% and climbing.

It's a dead-simple Ruby/JS app with zero server-side storage, hence the lack of historical data. Source code here: https://github.com/jpatokal/untether