'Create a combination chart with a secondary chart.
Note: Override parent method to add an extra check that is required
for Bar charts to ensure that their combined chart is on a secondary
axis.
Args:
chart: The secondary chart to combine with the primary chart.
Returns:
Nothing.'

def combine(self, chart=None):

    if (chart is None):
        return
    if (not chart.is_secondary):
        warn('Charts    combined    with    Bar    charts    must    be    on    a    secondary    axis')
    self.combined = chart
