Pie sectors default to a thin black line as the sector edge. You can use this property to remove the edges completely, or set a differently coloured (maybe thicker) line as required:

chSet 'edge' 'white,solid,2'

This works well with ‘inside’ value labels as shown in the example. Only solid edges are supported at present, although other edge-styles are accepted here. To remove the edges completely, use

chSet 'edge' 'none'

This sets the line-weight to 0, so no edge is drawn.