Bandwidth distribution graph has "0 Gbit/s" for all y axis labels

See e.g. https://metrics.torproject.org/advbwdist-perc.html?start=2015-01-19&end=2020-04-18&p=50

This is likely the same type of rounding bug as legacy/trac#33066 (moved).

(I wonder if there are other graphs that have this issue, too?)