A lichen survey

Lichen species were counted on the trees of eight places (the numbers are made up for this lab). You will write a function that grades the air from a count, grade every place, and count the places in each grade. Then the program draws the counts.

The idea. Lichens do not grow where the air is polluted. Take $8$ or more species as clean air, $4$ to $7$ as moderate, and fewer than $4$ as polluted.

Your tasks.

1. In grade, return "clean", "moderate" or "polluted" for a number of species.
2. Set grades to the grade of each place, in the order of places.
3. Set tally to a dictionary of each grade and the number of places with it.