[philh]

Oh, I somehow missed that half of the intervals were only 20 years.

But it's not a parametric plot either, because the number of pirates goes up and then down.

I guess it could plausibly be a normal graph with the X axis being some silly function of time; or a parametric plot with the X axis being a stupid function of pirates. But really I think aw3c2 was right: the graph makes zero sense.

[sesqu]

Here's a better version of the plot: http://i.imgur.com/aXQei.png

This type of plot can be confusing when the relationship isn't strong, but I think it works well enough here.

edit: there's also this plot type: http://i.imgur.com/jgMFS.png

[SimHacker]

It's a parody of religious right wing magical thinking, so it's not supposed to make sense.