make histogram analysis json serializable

3 jobs for master in 38 minutes and 5 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build Sdist
passed #29656
build:sdist

00:00:21

 
  Build Image
passed #29657
privileged
build:image

00:33:33

 
  Test
failed #29658
privileged
test

00:04:10

81.0%
 
Name Stage Failure
failed
test Test
  A value is trying to be set on a copy of a slice from a DataFrame.
Try using .loc[row_indexer,col_indexer] = value instead

See the caveats in the documentation: http://pandas.pydata.org/pandas-docs/stable/indexing.html#indexing-view-versus-copy
df['value'] = transformations[transformation](df['value'])

-- Docs: http://doc.pytest.org/en/latest/warnings.html
======== 263 passed, 10 skipped, 10 warnings, 1 error in 100.12 seconds ========
ERROR: Job failed: exit code 1