"No data" for XY graph when using interator with Python 3

* Using a `list()` cast avoids the problem.
* not seen with Python 2.7
* plotting two lines, one with iterator and one with list, plots both lines which is confusing

Test case:

#!/usr/bin/env python
# -*- coding: us-ascii -*-
# vim:ts=4:sw=4:softtabstop=4:smarttab:expandtab
# Python 2 or 3
# Bug https://github.com/Kozea/pygal/issues/499
# "No data" for XY graph when using interator with Python 3

import sys

import pygal

print(sys.version)

t_chart = pygal.XY(stroke=True)
t_chart.title = 'interator bug'
#t_chart.x_title = 'Time (seconds)'
#t_chart.y_title = 'Temperature (C)'
t_chart.truncate_legend = 30 # or -1 to disable

# NOTE Python 3.8 needs list cast, iterator does not work
t_chart.add('fails py3 (works py2)', zip(range(1, 10), range(100, 110))) # Py3 shows "No data", unless below is also ran
#t_chart.add('works', list(zip(range(5, 15), range(100, 110)))) # work around for Py3 - NOTE if doing both of these, fails suddenly works and both lines are displayed

filename = 'chart.svg'
t_chart.render_to_file(filename) # py2 and py3
print('check file %s' % filename)

Start with the Python 3 reproducer in the issue, especially pygal.XY.add() with a zip iterator and render_to_file(). Compare the iterator behavior with the list() workaround and confirm the expected result is that the XY graph displays its data without requiring callers to materialize the iterator.

Written by the indexing model from the issue text.