So this works and it creates pretty graphs:
But it's pretty inefficient as an Iron.io worker and will end up being pretty expensive.
The Iron.io worker was abandoned and now it works as a small Sinatra app that you can deploy on Heroku.
You can adjust the Sendgrid Events webhook from this page.
curl -XPOST -H 'Content-Type:application/json' -H 'Accept: application/json' --data-binary @sendgrid.json http://localhost:4567/sendgrid/ -v -s
The Heroku deploy button may not work, but push the code to Heroku and setup your required environment variables and you'll be off to the races.