Regression in Histogram axis labeling

[jpallas]

At first I thought this might be something in my dev environment, but I reproduced it using Binder with the latest master.

The second cell of doc/groovy/Histogram.ipynb is coming out like this:

It used to come out like this:

Something happened to the tick labels on the axes.

[jpallas]

git bisect is pointing at c497669, which is certainly a plausible suspect.