As described above vertical positions are specified as a numeric value [0..n] where 'n' is an arbitrary constant. (For practical purposes n is most likely < 100)
Using our previous example we will illustrate this parameter by changing the position of our 'Project' activity to position 7. Therefor we change the call to GanttBar() to
$activity = new GanttBar( 7,"Project","2001-12-21" ,"2002-02-20" );
and we then get the chart as shown below in Figure 146.
Note that the height of each position (vertical position) will depend on the actual height of the bar.