feature request - give edge 'background' color

[dczielinski]

So, this may be specific to my use case, but I'm trying to make maps that maximize interconnectivity, so you can visualize it easily. However, some reaction edges end up having to cross over a signficant portion of the map. It would be much more visually appealing if I could 'gray them out' somehow, where just like toggling metabolite nodes from big to small, I could toggle particular reaction edges from primary (bold blue) to secondary (gray or partially transparent blue).