title: Creating arrow head matching size (or lwd) in ggplot2
body: <p>To draw arrows in ggplot, I use geom_segment and arrow=arrow().
  I would like the arrow head size to match the segment width (or size).
  However, arrow does not recognize variables directly from the data argument in ggplot and one must specify data.frame containing the variable using the $ operator. This causes a disjunct between the values used for plotting the line and those used for plotting the arrow head (the largest arrow head can be on the thinest segment).</p>
  
  <p>Example:</p>
  
  <pre><code>d &lt;- structure(list(Process = structure(c(2L, 1L, 1L, 1L, 2L, 2L, 
  1L, 1L, 2L, 1L, 2L), .Label = c("First", "Second"), class = "factor"), 
  x.sink = c(1, 3, 1, 2, 2, 3, 3, 2, 2, 2, 2), y.sink = c(1, 
  1, 1, 2, 2, 1, 1, 1, 1, 2, 2), x.source = c(2, 2, 2, 2, 2, 
  2, 2, 1, 1, 1, 3), y.source = c(2, 2, 2, 1, 1, 1, 1, 1, 1, 
  2, 1), offset = c(1, 1, 1, -1, -1, -1, -1, -1, -1, 1, -1), 
  Std.Flux = c(0.179487179487179, 0.170940170940171, 0.944444444444444, 
  0.0854700854700855, 0.726495726495726, 0.128205128205128, 
  0.213675213675214, 0.213675213675214, 0.128205128205128, 
  0.106837606837607, 1)), .Names = c("Process", "x.sink", "y.sink", 
  "x.source", "y.source", "offset", "Std.Flux"), class = "data.frame", row.names = c(NA, 
  -11L))
  
  
  p &lt;- qplot(data=d,
             #alpha=I(0.4),
             colour=Process,
             size=Std.Flux,
             xlim=c(0,4),
             ylim=c(0,3),
             x=x.source+as.numeric(Process)/10,
             y=y.source+as.numeric(Process)/10,
             xend=x.sink+as.numeric(Process)/10,
             yend=y.sink+as.numeric(Process)/10,
             geom="segment",
             arrow = arrow(type="closed",
                           length = unit(d$Std.Flux,"cm")))
  print(p)
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/1bEY6.jpg" alt="Disjunct arrow heads"></p>
  
  <p>Any suggestions?</p>