Combine phylogenetic tree with x,y graph

I'm trying to arrange a phylogenetic tree onto a graph showing physiological data for a set of related organisms. Something like the picture below. This was put together in powerpoint from 2 separate graphs. I guess it gets the job done, but I was hoping to create a single image which I think will be easier to format into a document. I am able to produce the graph I want using ggplot2, and import the tree using ape. I was thinking there should be a way to save the tree as a graphical object and then arrange it with the graph using the gridarrange function in gridExtra. The problem is that ape won't let me save the tree as a graphical object, e.g.,

p2<-plot(tree, dir = "u", show.tip.label = FALSE)

just plots the tree and when you call p2 it just gives a list of arguments. I'm wondering if anyone has any tips.

Thanks!

I'm not sure if that will work with gtable from CRAN

require(ggplot2)
require(gridBase)
require(gtable)

p <- qplot(1,1)
g <- ggplotGrob(p)

g <- gtable_add_rows(g, unit(2,"in"), nrow(g))
g <- gtable_add_grob(g, rectGrob(),
                     t = 7, l=4, b=7, r=4)

grid.newpage()
grid.draw(g)

#grid.force()
#grid.ls(grobs=F, viewports=T)
seekViewport("layout.7-4-7-4")
par(plt=gridPLT(), new=TRUE)
plot(rtree(10), "c", FALSE, direction = "u")
upViewport()