DateY chart issues w/single datapoint

[jonapich]

The DateY chart seems to be missing some hooks when there's only 1 datapoint to show.

For instance, rendering a DateY chart with a single data point will render an empty graph (without the "NoData" text). The single datapoint will not be visible.

Worst: When Interpolation is enabled and there's only a single datapoint, render() will raise an AttributeError:

AttributeError                            Traceback (most recent call last)
<ipython-input-4-97a5b0c16273> in <module>()
----> 1 bmark.chart_document_count

E:\Projects\Repos\TestAutomation\pyces\sandbox\benchmarks\benchmarker.pyc in chart_document_count(self)
     59         for name, data in self.results.items():
     60             chart.add(name, data)
---> 61         return SVG(chart.render())
     62 
     63     def start(self):

C:\Apps\Python\lib\site-packages\pygal\ghost.pyc in render(self, is_unicode, **kwargs)
    109     def render(self, is_unicode=False, **kwargs):
    110         return (self
--> 111                 .make_instance(overrides=kwargs)
    112                 .render(is_unicode=is_unicode))
    113 

C:\Apps\Python\lib\site-packages\pygal\ghost.pyc in make_instance(self, overrides)
    103         self._last__inst = self.cls(
    104             self.config, series, secondary_series, self.uuid,
--> 105             self.xml_filters)
    106         return self._last__inst
    107 

C:\Apps\Python\lib\site-packages\pygal\graph\line.pyc in __init__(self, *args, **kwargs)
     31     def __init__(self, *args, **kwargs):
     32         self._self_close = False
---> 33         super(Line, self).__init__(*args, **kwargs)
     34 
     35     @cached_property

C:\Apps\Python\lib\site-packages\pygal\graph\base.pyc in __init__(self, config, series, secondary_series, uuid, xml_filters)
     67 
     68             self.zero = min(positive_values or (1,)) or 1
---> 69         self._draw()
     70         self.svg.pre_render()
     71 

C:\Apps\Python\lib\site-packages\pygal\graph\base.pyc in _draw(self)
    281         self._decorate()
    282         if self.series and self._has_data():
--> 283             self._plot()
    284         else:
    285             self.svg.draw_no_data()

C:\Apps\Python\lib\site-packages\pygal\graph\line.pyc in _plot(self)
    167     def _plot(self):
    168         for serie in self.series:
--> 169             self.line(serie)
    170 
    171         for serie in self.secondary_series:

C:\Apps\Python\lib\site-packages\pygal\graph\line.pyc in line(self, serie, rescale)
    121         if serie.stroke:
    122             if self.interpolate:
--> 123                 points = serie.interpolated
    124                 if rescale and self.secondary_series:
    125                     points = self._rescale(points)

AttributeError: 'Serie' object has no attribute 'interpolated'

Everything starts to work again when there are at least 2 datapoints to show.

[paradoxxxzero]

Fix with DateLine in 1.7.0