Python-pptx line chart error - python-2.7

Question

Newb here just getting into Python and ran into an issue that's beating me down. I have the following excerpt of Python code to create a PPT slide from an existing template. The layout and placeholders are correct but I can't get it to run with my data listed below (x, y_in, & y_out). Any help is greatly appreciated.
x = [datetime.datetime(2017, 8, 4, 15, 5, tzinfo=<FixedOffset u'+00:00' datetime.timedelta(0)>), datetime.datetime(2017, 8, 4, 15, 10, tzinfo=<FixedOffset u'+00:00' datetime.timedelta(0)>), datetime.datetime(2017, 8, 4, 15, 15, tzinfo=<FixedOffset u'+00:00' datetime.timedelta(0)>), datetime.datetime(2017, 8, 4, 15, 20, tzinfo=<FixedOffset u'+00:00' datetime.timedelta(0)>)]
y_in = [780993, 538962, 730180, 1135936]
y_out = [5631489, 6774738, 6485944, 6611580]
prs = Presentation('Network_Utilization_template_master.pptx')
slide = prs.slides.add_slide(prs.slide_layouts[2])
placeholder = slide.placeholders[17]
chart_data = CategoryChartData()
chart_data.categories = x
chart_data.add_series(y_in)
chart_data.add_series(y_out)
graphic_frame = placeholder.insert_chart(XL_CHART_TYPE.LINE, chart_data)
chart = graphic_frame.chart
chart.has_legend = True
chart.legend.include_in_layout = True
chart.series[0-2].smooth = True
prs.save("Network_Utilization_" + today_s + ".pptx")
the compiler spits out the following:
Traceback (most recent call last):
File "/Users/jemorey/Documents/pptx-2.py", line 81, in <module>
graphic_frame = placeholder.insert_chart(XL_CHART_TYPE.LINE, chart_data)
File "/Users/jemorey/Library/Python/2.7/lib/python/site-packages/pptx/shapes/placeholder.py", line 291, in insert_chart
rId = self.part.add_chart_part(chart_type, chart_data)
File "/Users/jemorey/Library/Python/2.7/lib/python/site-packages/pptx/parts/slide.py", line 174, in add_chart_part
chart_part = ChartPart.new(chart_type, chart_data, self.package)
File "/Users/jemorey/Library/Python/2.7/lib/python/site-packages/pptx/parts/chart.py", line 29, in new
chart_blob = chart_data.xml_bytes(chart_type)
File "/Users/jemorey/Library/Python/2.7/lib/python/site-packages/pptx/chart/data.py", line 104, in xml_bytes
return self._xml(chart_type).encode('utf-8')
File "/Users/jemorey/Library/Python/2.7/lib/python/site-packages/pptx/chart/data.py", line 128, in _xml
return ChartXmlWriter(chart_type, self).xml
File "/Users/jemorey/Library/Python/2.7/lib/python/site-packages/pptx/chart/xmlwriter.py", line 803, in xml
'ser_xml': self._ser_xml,
File "/Users/jemorey/Library/Python/2.7/lib/python/site-packages/pptx/chart/xmlwriter.py", line 902, in _ser_xml
'tx_xml': xml_writer.tx_xml,
File "/Users/jemorey/Library/Python/2.7/lib/python/site-packages/pptx/chart/xmlwriter.py", line 191, in tx_xml
'series_name': self.name,
File "/Users/jemorey/Library/Python/2.7/lib/python/site-packages/pptx/chart/xmlwriter.py", line 121, in name
return escape(self._series.name)
File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/xml/sax/saxutils.py", line 32, in escape
data = data.replace("&", "&")
AttributeError: 'list' object has no attribute 'replace'

Answer 1

David Zemens is quite right in his comment. A series has a name, which appears as the first argument to ChartData.add_series(). The name appears in the legend next to the line color for that series and also appears as the column heading for the data for that series. Adding that in should get you to your next step.
Something like:
chart_data.add_series('MB in', y_in)
chart_data.add_series('MB out', y_out)