Add some reference for helping people interpret NO2 levels

[patudom]

A focus group educator pointed out that learners would need help understanding what the numerical values of NO2 mean - what is healthy or unhealthy?

We can try adding a colored background to the time graph (that can maybe be toggled on/off) that shows green/yellow/red levels of healthiness based on EPA AQI level guidelines.

• Ask Caroline to help us determine what those levels should be.