Using penwidth with multi-color edges

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1

The behavior of the penwidth attribute on multi-color edges is somewhat unexpected. As it increases beyond penwidth=2, only the last color widens.

This source:

graph G {
a -- b [color="red:green:blue:orange" penwidth=1]
c -- d [color="red:green:blue:orange" penwidth=2]
e -- f [color="red:green:blue:orange" penwidth=6] 
}

renders as:

[dot]

graph G {
a -- b [color="red:green:blue:orange" penwidth=1]
c -- d [color="red:green:blue:orange" penwidth=2]
e -- f [color="red:green:blue:orange" penwidth=6] 
}

[/dot]

Presumably the lines are being drawn overlapping, with the pen only moving 2 units to the right each time. But is there any way to control that, and get the lines to be both thicker and evenly-spaced?