I just zoomed in at 200 % and found things to be quite correct. 02/01 was a sunday and the gridline is set just between the data. Also Feb has had only 29 days and the 03/01 minor top of 2057 lies exactly on the grid. 04/01 is also displayed correctly.
If you zoom in to 400 % it seems that the grids even lie between days 31 and 01.
In any case it should be sufficient, there's no daily map. (Glad about that <g>)