• Simple layout. It's easy to see you have days within months going left to right and years going down.
• Colors match the official EPA colors for the AQI categories
• Including the ranges we know what constitutes a good reading vs. a bad reading
• The title tells me what I'm looking. Including the date range is a nice touch too.
• Using a strip plot along with the colors helps reveal seasonal patterns
• Including the source in the footer

What could be improved?

• Make the colors color-blind friendly
• It's difficult to see how a month has changed across the years.
• I can't hover to see the values.
• It says each “tile” represents one day of the year and is color-coded based on the AQI level for that day, but I think it's actually showing the max measurement for each day. It should be more clear what they are representing.
• There's very little context. How does one county compare to others? How does one county compare to the national average? 

What were my goals?

• Last year I challenged The Data Duo to visualise an almost identical data set. Pooja created a pretty amazing viz (of course), so I wanted to pick some parts off of it. Particularly, the State selectors on each site and the dot plot.
• I wanted to understand how each month changed across the years, which is pretty much exactly what a cycle plot was created for. I used a trend line instead of an average though because that seemed to show the patterns better.
• I wanted to add context for comparisons to the national average and to the state average (which appears when you click on a state).
• I wanted to use color-blind friendly colors.
• I wanted a small sparkline-type chart to show how things have changed since 1990.