I want to be able to create a curved 4-sided polygon with equal perpendicular widths, like a thick bezier curve:

Click image for larger version.  Name: line_equal_widths.png  Views: 390  Size: 10.4 KB  ID: 93569

... but I want to do that with a shape, adding the curves, one at a time.

So, say I have "Curve 1". I want to create "curve 2" such that along any point on either curve, the perpendicular distance between the nearest point is equal.

You can't just copy and paste the same curve at a 45 degree angle, it bulges in the middle, like so:

Click image for larger version.  Name: curve_bulge.jpg  Views: 326  Size: 19.9 KB  ID: 93570


The reason I want to do this is to create what is called a "sankey diagram", which shows quantitative flows, like the following image:

Click image for larger version.  Name: sankey_sample.jpg  Views: 371  Size: 15.4 KB  ID: 93571

I want to be able to do a really nice one, like so:
http://www.wri.org/image/view/11147/_original

There is a decent sankey tutorial I found using the stacked column graph tool, but it doesn't seem to use curved lines.
http://www.nelsonelson.com/wiki/inde...ankey_Tutorial

Of course, I could approximate things by using bezier curves, but I can be much more exact by using separate curves.

Any help will be very greatly appreciated :)