total_cost
date
2006-03-04 -1465.052092
2006-04-04 -1213.508277
2006-05-04 -1459.290503
2006-06-04 -1460.119361
2006-07-04  -772.482609

[2006,2007,2008,2009,2010,2011,2012]

[1,2,3,4,5,6,7,8,9,10,11,12]

year    month      cost
2006        1     12345
2006        2     12345
:                     :
2012       12     12345