Bars and Milestones need both a vertical position and a horizontal position. The horizontal start position is specified as a date, e.g. "2001-06-23", and the vertical positions are specified as a number [0,1,2,3,...]. This vertical number indicates the position from the top where the object should be placed. To understand this you might imagine a number of "invisible" horizontal bands with a certain height. If you specify 0 as the vertical position the bar will be placed in the first band, specify 3 and the bar will be placed in the fourth band and so on.
It is perfectly legal, and perhaps even desirable to leave "gaps" when laying out bands to group related activities. So, for example you could have three activities/bars at positions 1,2,3 and then another 2 bars at position 6,7 leaving band 0,4,5 empty.
All these "invisible bands" have the same height (equ-spaced). The height of each band is automatically determined and depends on both the method of layout ( as specified by (GanttChart::SetLayout()) and the individual heights of the individual bars and titles. The rules are quite simple: