Incubator-superset: Time-series Period Pivot - DateOffset objects are immutable

[AndLLA]

Time-series Period from trunk is no more functional (used to work a couple of weeks ago),
The chart fails with the stack trace below.

Traceback (most recent call last):
File "/home/superset/venvSS/lib/python3.6/site-packages/superset/views/base.py", line 121, in wraps
return f(self, args, kwargs)
File "/home/superset/venvSS/lib/python3.6/site-packages/superset/utils/decorators.py", line 70, in wrapper
return f(args, *kwargs)
File "/home/superset/venvSS/lib/python3.6/site-packages/superset/views/core.py", line 1119, in explore_json
viz_obj, csv=csv, query=query, results=results, samples=samples
File "/home/superset/venvSS/lib/python3.6/site-packages/superset/views/core.py", line 1046, in generate_json
payload = viz_obj.get_payload()
File "/home/superset/venvSS/lib/python3.6/site-packages/superset/viz.py", line 379, in get_payload
payload["data"] = self.get_data(df)
File "/home/superset/venvSS/lib/python3.6/site-packages/superset/viz.py", line 1401, in get_data
freq.normalize = True
File "pandas/_libs/tslibs/offsets.pyx", line 332, in pandas._libs.tslibs.offsets._BaseOffset.__setattr__
*AttributeError: DateOffset objects are immutable.

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[villebro]

It seems DateOffset has been made immutable in pandas 0.24, and there was is no obvious solution to making the normalize attribute True. Let me think about this for a sec to see how it can be solved.

[AndLLA]

The chart is now working,
however in my setup the results are strange (time grain=day, last quarter, metric=count(*), freq=1week start monday)
The chart shows all points stuck on the first day of the week (no lines).

[villebro]

FYI @betodealmeida the fix only seems to work for certain offsets, reopening this. Posted the following on Pandas issue tracker: https://github.com/pandas-dev/pandas/issues/27728