Finding a variety of charts on Trulia is very straightforward. Just navigate on over to Local Info on the Green Banner above and select Cos Cob Market Trends. Then follow the instructions or visit the "Help Center" to find the answer. Trulia's Help Center link is usually at the bottom right of every page.
BTW, often times in smaller markets the median selling price of homes is a better perspective since one or two homes sold can skew the average.
Good luck! I'm reasonably certain you will accomplish your goal.... more