Plot daily AQI values for a specific location and time period. Each square or “tile” represents one day of the year and is color-coded based on the AQI level for that day. The legend tallies the number of days in each AQI category.
They also make all the data available as a CSV file for each year, so one can compute the daily/weekly/monthly trend by averaging over several years.
airnow.gov gives the air quality for a given day in different locations in a US state in the past. Downside: doesn't give directly a clear daily/weekly/monthly trend.