Race condition / itermittent failure in one of the barplot tests

Noticed while working on #521 -- [this particular test](https://github.com/biocore/empress/blob/bf6f755c5187e543ff13c2c624b153fb97918b23/tests/test-barplots.js#L353-L359) was failing (it was expecting `216` coordinates, but only saw `72`). Adding log statements during debugging seemed to cause the test to stop failing, and the test kept passing even after I removed the logs again?

Since I don't think #521 touches any of the code impacting this check, I suspected at first that it was a race condition, but now I'm not sure about that. In any case, we can always comment this test out if it becomes a problem, since it's not super important.

Start with tests/test-barplots.js lines 353-359 and run the referenced barplot test repeatedly to reproduce the intermittent 216-versus-72 coordinate failure. Trace the test and the code it exercises to determine whether execution order or another condition causes the mismatch. Done means the test passes deterministically with the intended assertion, or the issue documents a confirmed cause and appropriate resolution.

Written by the indexing model from the issue text.