Users 1 and 15's charts make no sense - they have to be special system accounts of some sort. My guess is that #15 is the account that receives trading fees, and #1 represents MtGox itself (or some specific aspect of MtGox, such as its cold-storage).

There're still a lot of points on these plots that don't make sense, though; generally they look like vertical stripes labelled as sets of small sell orders, both far below and far above the market price. User 30 has what looks like a large sell (in Mar 2013) far above the highest-ever price. So either MtGox's order-matching is way more broken than anyone ever knew (unlikely), or these actually represent fees, withdraws, or something similar, and the y-axis position is meaningless.

Also worthy of note is that massive sell order by user 1 in Nov 2013. That's hard to interpret without knowing what the dots on that graph really mean (I doubt they're trades), but it's likely significant.