Made datetime handling in plotting code more general

[philippjfr]

Improves datetime handling for datetime.datetime and pd.tslib.Timestamp types. Needs a bunch of unit tests.

[philippjfr]

Very happy with this refactor the code is no longer duplicated and now handles datetime types more consistently. Also added a bunch of unit tests just now. Ready to merge when tests pass.

[jlstevens]

Ok, the tests have now passed. The changes look good and I am very happy to see the code simplified and the addition of new tests.

Merging.

[stonebig]

hi small remark. testing with holoviews latest build, I get this error:

• holoviews-1.7.dev8-py3-none-any.whl
• pandas-0.20.0rc1-cp35-cp35m-win_amd64.whl

# Holoviews 
# for more example, see http://holoviews.org/Tutorials/index.html
import holoviews as hv
%load_ext holoviews.ipython
fractal = hv.Image(image)

((fractal * hv.HLine(y=0.16)).hist() + fractal.sample(y=0.16))

C:\WinPython\basedir35\buildQt5\winpython-64bit-3.5.x.2\python-3.5.3.amd64\lib\site-packages\holoviews\core\util.py:26: FutureWarning: pandas.tslib is deprecated and will be removed in a future version.
You can access Timestamp as pandas.Timestamp
  datetime_types = datetime_types + (pd.tslib.Timestamp,)

..; and the image is not displayed (on Jupyterlab-0.20.0), but that may not be an holoviews problem

[philippjfr]

Thanks for the note, I'll make a PR with the fix now.

[stonebig]

further down, i see errors in statsmodels "The pandas.core.datetools module is deprecated and will be removed in a future version. Please use the pandas.tseries module instead.
from pandas.core import datetools"... maybe a full check would be nice

[philippjfr]

further down, i see errors in statsmodels "The pandas.core.datetools module is deprecated and will be removed in a future version. Please use the pandas.tseries module instead.
from pandas.core import datetools"... maybe a full check would be nice

Could this be triggered by some other dependency? I think it's probably seaborn, because holoviews does not import statsmodels directly.

[stonebig]

it's not triggered by holloviews, just meant that there may be a lot more deprecation warnings to chaze per holoviews with pandas-0.20.0rc1

[philippjfr]

I see, yes that makes sense, I'll try it out.

[stonebig]

oups ! it's a new kind of deprecation for me "FutureWarning". I new "DeprecationWarning" and "UserWarning" up to now.

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