timestamp
2010-05-01      2
2010-06-01      1
2010-07-01      2
2010-08-01      3
2010-09-01      0
2010-10-01      5

timestamp
2010-02-01     0
2010-03-01     0
2010-04-01     0
2010-05-01     1
2010-06-01     1
2010-07-01     2
2010-08-01     3
2010-09-01     0
2010-10-01     2

timestamp
2010-02-01     0
2010-03-01     0
2010-04-01     0
2010-05-01     2
2010-06-01     1
2010-07-01     1
2010-08-01     1
2010-09-01     0
2010-10-01     2