def __getitem__(self, dim):
        if dim in self._selections:
            return self._selections[dim]
        else:
            return None
For now, just use a sequential implementation for reductions
dataset/ETHPy150Open bokeh/bokeh/bokeh/charts/data_source.py/ChartDataSource.__getitem__--crossover with--dataset/ETHPy150Open iskandr/parakeet/parakeet/openmp_backend/multicore_compiler.py/MulticoreCompiler.visit_IndexReduce