Certainly an interesting idea and neat to look at, though I think there's room for improvement in making the data more useful.

Firstly, the current ranking method is very susceptible to large fluctuations as it is based on exactly a week period; there's no cumulative or rolling effect. This is quite apparent when you look at the large number of "Last Week: N/A" that are present every single week. Any given week - on its own - is a decent snapshot of activity, but trying to plot any trends is difficult when you can see an item shoot into the #2 spot from "N/A" one week, then be gone from the rankings the week after. I would suggest something like a rolling daily ranking of the past X days' activity, or having historical rankings contribute to the current rankings with a decaying relevance over time. This would hopefully stabilize the rankings, and make more useful trend analysis possible.

Second, a graph to plot the movement of the plugins/themes on the chart would also be very handy. Having to go back and load each week to check a particular item's progress is quite time consuming. Though the ranking volatility would need to be addressed first; such a graph wouldn't be particularly useful with items constantly jumping on and off the rankings week to week.