I saw this data in a Tweet from Erica Barnett (above) about the relationship between Seattle (King County) homelessness and rental prices. This data fits pretty well with a dynamic information equilibrium  (i.e. "matching" model per my recent paper):
There was a shock that reduced rental prices during the Great Recession (centered at 2008.9, lasting about 6 months). This temporarily halted the growth in rental prices, which was followed by a temporary halt to homelessness growth (a shock centered at 2011.2, lasting about 2 years). Since those shocks, rental prices and homelessness have returned to their previous trajectories. Absent some kind of intervention, it is forecast to continue.
 Although I call this an equilibrium, it doesn't mean this equilibrium is "good" for social welfare. The fact that rental prices and homelessness are directly connected is more a failure of the market provision of housing than an endorsement.