By default, all bars are edged in black with a 0.4pt line. To change this (or remove the edge)

chSet 'bedge' 1.2 
chSet 'be' 0

Unedged bars can be used to show horizontal or vertical bands:

This example uses an unedged Gantt chart to show the safe zone for the cumulative deviation from the norm. It can be found in the Thumbnails2 example.