flexible position of tickmarks on x-axis

[mfaber]

tickmarks on the x-axis are always drawn to the right of the respective bars. This can be confusing because tickmarks are then actually "between" two days.

It would be great to have some flexibility (e.g., to have tickmarks centered below the respective bar). Does not need to be the default, but as it is according to the style of many epi-journals and also EPIET, it would be great to have this option. Thanks :)