Add AQI Plot

[caspar]

Calculate the AQI using pollutant concentration data in
the following equation:
https://www.epa.gov/sites/production/files/2014-05/documents/zell-aqi.pdf
PMobs = observed 24-hour average concentration in µg/m3
PMmax = maximum concentration of AQI color category that contains PMobs
PMmin = minimum concentration of AQI color category that contains PMobs
AQImax = maximum AQI value for color category that corresponds to PMobs
AQImin = minimum AQI value for color category that corresponds to PMobs