294: Improve breaks and bins for probability chart

[cincodenada]

For probability, I think breaks of 25 are more natural, and that
revealed that the bin at 50 was 45-50, which was unintuitive, so I set
an offset so that the bins are centered on multiples of 5, instead of
those being the borders.

[cincodenada]

After looking more closely, there are a few ways to accomplish the same bins - bins=21 will also do it, so if that seems nicer, go for it. I think a bin centered on the breaks tells the story best, and it appears 21 bins are required to have a bin width of 5 to accomplish that, because the default is to center the bins on the min/max values, so there's an "extra" bin (since the first/last bins are each only half a bin). The effective binwidth in the existing chart is actually 5.26, I believe.