time       Y          item price
2009-01-01 2.45206090 X    0.65473572
2009-01-01 2.45206090 Z    -1.46570942
2009-01-02 0.21547130 X    -0.89248112
2009-01-02 0.21547130 Z    -2.05357735
2009-01-03 1.31792870 X    0.98012375
2009-01-03 1.31792870 Z    4.64248357
2009-01-04 3.37656450 X    0.35397376
2009-01-04 3.37656450 Z    -0.25237774
2009-01-05 1.34775110 X    2.19357813
2009-01-05 1.34775110 Z    0.09719105
