Three years of monthly lawn$-$maintenance expenses $($$$)$ for a six$-$unit apartment house in southern Florida follow.

$\backslash$table$[[$Month$,$Year $1,$Year $2,$Year $3],[$January$,170,180,195],[$February$,180,205,210],[$March$,205,215,230],[$April$,230,245,280],[$May$,240,265,290],[$June$,315,330,390],[$July$,360,400,420],[$August$,290,335,330],[$September$,240,260,290],[$October$,240,270,295],[$November$,230,255,280],[$December$,195,220,250]]$

What type of pattern exists in the data?