The first in document order, as in top-to-bottom.
I’m not sure why the bar chart is getting selected in that notebook. Our thumbnail daemon isn’t great with WebGL (and… many other non-trivial things), or it may be the thirty-second timeout.
We have plans to ship a better thumbnail daemon, and we also want you to be able to select a thumbnail explicitly when publishing. But neither of those features are our highest priority right now, so you might have to put a static hero image at the top of your notebook if you want to guide the thumbnailer in the right direction in the interim.