Issues With Zero Values

[OptrixAU]

I've got a scenario where some nodes or even entire columns will have zero values, and this appears to cause issues.

Specifically, when calculating the ky factor in initializeNodeBreadths. If you have a ky of zero, no values can be drawn and all lines are minimum thickness.

To resolve this, columns with a zero total should be ignored.

It's one of the issues resolved in https://github.com/OptrixAU/d3-sankey