[BioC] Rgraphviz: adding weight to an edge

[Jeff Gentry]

>  edgeA$weight["a~b"] <- 10       # here comes the buggy
>  edgeA$weight["b~c"] <- 20       # one more
>  If we dont use the weights it works fine. Is this a bug, or I am
> doing something stupid?

Well, not that this excuses the segfault (it shouldn't happen, I'll look
at this) but you are definitely setting weights in an odd manner.  Why not
just set them on the graph itself?