Stacking multiple plots vertically with the same X axis but different Y axes

library(FinCal)
library(reshape2)

dat <- get.ohlc.yahoo('AAPL', '2015-12-01', '2015-12-31')
#dat$date <- strptime(dat$date, "%Y-%m-%d")
dat$date <- as.Date(dat$date, "%Y-%m-%d")
dat$times <- seq(nrow(dat))
mm <- melt(subset(dat, select=c(times,adjusted, volume)), id.var="times")

library(lattice)
xyplot(value~times|variable,data=mm,type="l",
       scales=list(y=list(relation="free")),
       layout=c(1,2))

library(ggplot2)
ggplot(mm, aes(x = times, y = value)) + geom_line(aes(color = variable)) + 
      facet_grid(variable ~ ., scales = "free_y") + theme(legend.position = "none")

p1 <- ggplot(dat, aes(date, adjusted)) + geom_line() + theme_minimal() + 
      theme(axis.title.x = element_blank(), axis.text.x = element_blank())
p2 <- ggplot(dat,aes(date, volume)) + geom_bar(stat="identity") + theme_minimal() + 
      theme(axis.title.x = element_blank(),axis.text.x = element_text(angle=90))

library(grid)
grid.newpage()
grid.draw(rbind(ggplotGrob(p1), ggplotGrob(p2), size = "last"))

p3 <- p1 + scale_x_date(date_breaks = "1 day")
p4 <- p2 + scale_x_date(date_breaks = "1 day")
grid.newpage()
grid.draw(rbind(ggplotGrob(p3), ggplotGrob(p4), size = "last"))

require(gridExtra)
grid.arrange(p1, p2, ncol = 1, heights = c(2, 1))

gb1 <- ggplot_build(p1)
gb2 <- ggplot_build(p2)
n1 <- length(gb1$panel$ranges[[1]]$y.labels)
n2 <- length(gb2$panel$ranges[[1]]$y.labels)
gA <- ggplot_gtable(gb1)
gB <- ggplot_gtable(gb2)
g <- gtable:::rbind_gtable(gA, gB, "last")
panels <- g$layout$t[grep("panel", g$layout$name)]
g$heights[panels] <- list(unit(n1*5, "null"), unit(n2,"null")) # change 5 to other int
grid.newpage()
grid.draw(g)