zscore = df.dataflow([
df.joinaggregate().groupby(['symbol']).measure([df.mean('price').as('mean'), df.stdev('price').as('stdev')]),
df.formula('zscore', df.expr(d => (d.price - d.mean) / d.stdev).fields(['price', 'mean', 'stdev'])),
df.project(['symbol', 'date', 'zscore'])
])