'Test the _write_c_chart() method'

def test_write_c_chart(self):

    self.drawing._write_c_chart('rId1')
    exp = '<c:chart    xmlns:c="http://schemas.openxmlformats.org/drawingml/2006/chart"    xmlns:r="http://schemas.openxmlformats.org/officeDocument/2006/relationships"    r:id="rId1"/>'
    got = self.fh.getvalue()
    self.assertEqual(got, exp)
