XY chart gets bounding box wrong for incomplete data

[kevcampb]

If you do an XY chart and you supply multiple datasets, the bounding box for the y-axis may be wrong if some datasets define a value with a certain x-value, but other datasets don't.

line_chart = pygal.XY(width=1024, height=800)
line_chart.title = 'Bounding Box Issue'
line_chart.add("x1", [(10,10), (11,11), (12,12)])
line_chart.add("x1", [(10,11),          (12,13)])

The line chart will now include y=0

The issue appears to be at this section:

        if rng:
            self._box.xmin, self._box.xmax = min(xvals), max(xvals)
            self._box.ymin, self._box.ymax = min(yvals), max(yvals)

yvals will have 0 values for any dataset where an x-value exists in another dataset, but not that dataset.

[paradoxxxzero]

Indeed!
Thanks for the report.