This version of GANTT chart shows how you can use absolute dates to define start and end for segments, regardless of where the other segment in the same category ended.
The config uses <a href="http://docs.amcharts.com/3/javascriptcharts/AmGanttChart#startDateField"><code>startDateField</code></a> and <a href="http://docs.amcharts.com/3/javascriptcharts/AmGanttChart#endDateField"><code>endDateField</code></a> to point to a specific fields in segment data that hold their start and end dates respectively.
Please note that this demo also takes advantage of <a href="http://docs.amcharts.com/3/javascriptcharts/AmGanttChart#brightnessStep"><code>brightnessStep</code></a> setting. It allows increasing brightness with each subsequent segment gradually.