Typing quickly involves two main things:
(1) knowing where the keys are without looking
(2) typing them smoothly
Highly proficient typists have a steady rhythm to their keypresses but not many tools target this directly. Considering transitions between keys instead of standalone keys is a way to develop that rhythm and improve weakest links. Over the few days of developing and testing this I saw a 10% improvement in my WPM as well.
This is a static page with mostly custom javascript. The results are stored using localstorage for saving. I use charts.js for the bottom link. The data shown in the charts is exponentially smoothed.
The corpus of words is taken from Peter Norvig's lists of most common english words. I filter to the top 100,000 words. Then, for every pair, I try to grab the 500 most popular words within that. Several of the pairs don't have enough real words for them to be useful (ex. how many words can you think of that have an "qz" in them? ) I filter out profanity using zacanger's list.
I used Adam Schwartz's typing test as a template to get off the ground quickly.
Feel free to check out some of my other projects or reach out.