time       Y         item price
2009-01-01 2.4520609 X    0.65473572
2009-01-01 2.4520609 Z    -1.46570942
2009-01-02 0.2154713 X    -0.89248112
2009-01-02 0.2154713 Z    -2.05357735
2009-01-03 1.3179287 X    0.98012375
2009-01-03 1.3179287 Z    4.64248357
2009-01-04 3.3765645 X    0.35397376
2009-01-04 3.3765645 Z    -0.25237774
2009-01-05 1.3477511 X    2.19357813
2009-01-05 1.3477511 Z    0.09719105
