Describing buckets with arbitrary user-provided width.
.. attribute:: bounds
'bound' is a list of strictly increasing boundaries between
buckets. Note that a list of length N-1 defines N buckets
because of fenceposting. See comments on bucket_options
for details.
The i'th finite bucket covers the interval [bound[i-1],
bound[i]) where i ranges from 1 to bound_size() - 1. Note
that there are no finite buckets at all if 'bound' only
contains a single element; in that special case the single
bound defines the boundary between the underflow and
overflow buckets.
bucket number lower bound upper bound i == 0 (underflow)
-inf bound[i] 0 < i < bound_size() bound[i-1] bound[i] i ==
bound_size() (overflow) bound[i-1] +inf
