QGIS » QGIS Application

Especially when trying to view data on roads (such as vehicle counts per hour per segment), it's already very helpful to use automatic size scaling binded to a appropriate field. However, to proper visualize data according to direction of flow it's necessary to use some offset; ideally, the offset should be just enough to balance the line width out of center. Therefore I think It would be a huge improvement to allow the offset field to be set based on a field, in a way similar to size scaling.

If I'm not mistaken, the offset behavior should resemble
offset(field)=flat_offset*field/2
because we want
offset(field)=width(field)/2
, with width(field)=flat_width*field

Another way to tackle this would be render vector lines optionally off-center (to the right or to the left); than, the offset could remain as is. While this is generally the approach taken by other GIS/modelling software when showing this types of data (Inro Emme, PTV Visum and Caliper TransCAD take the render off-center (to the right) for these cases), I expected the QGIS way of doing to be similar to size scaling. And by allowing offset scaling by field the user would have much more flexibility...

I would very much like to implement this (since I have a particular use for it), but I would need some guidance on where things are/go on the source tree.