Your chart is a little hard to understand. What do the different symbols mean?
Check out the mt. gox one instead, with more trades, I think it is easier to understand:
http://sd-12155.dedibox.fr/~joel/btc/mtgox.png
I also have this new one, which is probably harder to understand but more useful as it automatically groups nearby trades:
http://sd-12155.dedibox.fr/~joel/btc/mtgox2.png
