added hack to make time pivot chart working

[srggrs]

guys

I noticed there is a bug in the chart time periodicity pivot. For example if you use the example birth data as the datasource/table ("birth_names), the chart return and error

'NoneType' object is not iterable

so I went down and dig in the code and realised there is some issue in getting the metrics values from metric. So I added that line below that make the chart working (try to use another data source with data time resolution of weeks or day to see the overlapping of all the lines as all the example data are with year minimum resolution hence can overlap them as there is no frequecy option higher than year).

I think the problem is that in the class NVD3TimePivotViz, the method query_obj can't override the default method that is in the base class BaseViz.

Am I right?

[mistercrunch]

Can you share the full stack trace you hit prior to this fix? At first sight I don't understand why your fix works.

[srggrs]

2018-09-19 16:44:44,467:DEBUG:root:�[36m[stats_logger] (incr) loaded_from_source�[0m
2018-09-19 16:44:44,467:ERROR:root:'NoneType' object is not iterable
Traceback (most recent call last):
  File "/home/serg/miniconda3/lib/python3.5/site-packages/superset/views/core.py", line 1105, in generate_json
    payload = viz_obj.get_payload()
  File "/home/serg/miniconda3/lib/python3.5/site-packages/superset/viz.py", line 359, in get_payload
    payload['data'] = self.get_data(df)
  File "/home/serg/miniconda3/lib/python3.5/site-packages/superset/viz.py", line 1381, in get_data
    df = self.process_data(df)
  File "/home/serg/miniconda3/lib/python3.5/site-packages/superset/viz.py", line 1147, in process_data
    values=utils.get_metric_names(fd.get('metrics')))
  File "/home/serg/miniconda3/lib/python3.5/site-packages/superset/utils.py", line 840, in get_metric_names
    return [get_metric_name(metric) for metric in metrics]
TypeError: 'NoneType' object is not iterable

error at line 44. Step to reproduce the error:
1 - from Sources >> Tables, select birth_names table
2 - after the query/explore page load, select TIme Series - Periodicity Pivot as the Visualization Type

[mistercrunch]

#5931 is a better fix

[srggrs]

@mistercrunch thank you for reviewing this. That was my first contribution in github and now I learned more how to post issues and pull request.

cheers,
serg