class TestReport(Report):
    _name = 'testreport.testreport'

    def __init__(self):
        super(TestReport, self).__init__()

    def parse(self, report, objects, datas, localcontext=None):
        datas['graph'] = [self.generate_graph(objects), 'image/png']
        return super(TestReport, self).parse(report, objects, datas,
                localcontext=localcontext)

[...]