r - Control Vertical margin when forcing y coordinates in riverPlot -

i'm trying plot diagram becomes narrow. automatic placement creates overlaying paths want avoid because paths simple , crossing complicates unncessarily, i'm forcing y-coordinates relative in riverplot specify what's on top , what's on bottom.

the problem vertical size of edges becomes small, small, nodes @ end of flow cannot seen properly.

see example in code below, when removing y coordinates edges thicker.

library(riverplot) riverplot(makeriver(   nodes = data.frame(     id = c("a", "b", "c"),     x = 1:3,     y = c(2,1,2)   ),    edges = data.frame(     n1 = c("a", "a"),     n2 = c("b", "c"),     value = c(100, 1)   ) )) 

i tried types of arguments passed riverplot not seem work, node_margin sounds should take care of doesn't seem when enforcing vertical positioning.

is there way decrease vertical node margin when doing allow thicker edges?