Chronograph analyzes which websites people come from when they click on a DOI.
When you click on a DOI you are redirected to the article landing page. We keep a log of every time a DOI link is followed (either by a human or by a machine), along with some standard data. This means that we can measure how many times each DOI is followed.
One standard piece of data that web browsers send is the 'Referrer header'. This indicates the website you were on when you clicked the DOI link. This allows us to know which websites are using DOIs and, of those, which DOIs people click. We don't include referrals from domains belonging to primary publishers, the members of Crossref who deposit links.
We also know whether the referrer was served over HTTP or HTTPS. Historically most web traffic used HTTP, but we're seeing increasing referrals from HTTPS.
Not all of the data in Chronograph is from humans: DOIs are resolved by automatic processes for a wide variety of reasons, but where the referrer is provided there's a good chance that there was a human involved.