def METHOD_NAME(self, dashboard):
    bsl = BaseStackedLine(x="key", y=["val"])
    bsl.chart = hv_dt.InteractiveDatashader()
    bsl.initiate_chart(dashboard)
    self.result = None
    def func1(data):
        self.result = data
    bsl.format_source_data = func1
    bsl.calculate_source(dashboard._cuxfilter_df.data)
    assert df_equals(self.result, dashboard._cuxfilter_df.data)