R, ggplot - Graphs sharing the same y-axis but with different x-axis scales

Extending your attempt #2, gtable might be able to help you out. If the margins are the same in the two charts, then the only widths that change in the two plots (I think) are the spaces taken by the y-axis tick mark labels and axis text, which in turn changes the widths of the panels. Using code from here, the spaces taken by the axis text should be the same, thus the widths of the two panel areas should be the same, and thus the aspect ratios should be the same. However, the result (no margin to the right) does not look pretty. So I've added a little margin to the right of p2, then taken away the same amount to the left of p2. Similarly for p1: I've added a little to the left but taken away the same amount to the right.

library(PtProcess)
library(ggplot2)
library(gtable)
library(grid)
library(gridExtra)

set.seed(1)
lambda <- 1.5
a <- 1
pareto <- rpareto(1000,lambda=lambda,a=a)
x_pareto <- seq(from=min(pareto),to=max(pareto),length=1000)
y_pareto <- 1-ppareto(x_pareto,lambda,a)
df1 <- data.frame(x=x_pareto,cdf=y_pareto)

set.seed(1)
mean <- 3
norm <- rnorm(1000,mean=mean)
x_norm <- seq(from=min(norm),to=max(norm),length=1000)
y_norm <- pnorm(x_norm,mean=mean)
df2 <- data.frame(x=x_norm,cdf=y_norm)

p1 <- ggplot(df1,aes(x=x,y=cdf)) + geom_line() + scale_x_log10() +
      theme(plot.margin = unit(c(0,-.5,0,.5), "lines"),
            plot.background = element_blank()) +
      ggtitle("pareto")
p2 <- ggplot(df2,aes(x=x,y=cdf)) + geom_line() + 
      theme(axis.text.y = element_blank(), 
            axis.ticks.y = element_blank(), 
            axis.title.y = element_blank(),
            plot.margin = unit(c(0,1,0,-1), "lines"),
            plot.background = element_blank()) +
      ggtitle("norm")

gt1 <- ggplotGrob(p1)
gt2 <- ggplotGrob(p2)

newWidth = unit.pmax(gt1$widths[2:3], gt2$widths[2:3])

gt1$widths[2:3] = as.list(newWidth)
gt2$widths[2:3] = as.list(newWidth)

grid.arrange(gt1, gt2, ncol=2)

EDIT To add a third plot to the right, we need to take more control over the plotting canvas. One solution is to create a new gtable that contains space for the three plots and an additional space for a right margin. Here, I let the margins in the plots take care of the spacing between the plots.

p1 <- ggplot(df1,aes(x=x,y=cdf)) + geom_line() + scale_x_log10() +
      theme(plot.margin = unit(c(0,-2,0,0), "lines"),
            plot.background = element_blank()) +
      ggtitle("pareto")
p2 <- ggplot(df2,aes(x=x,y=cdf)) + geom_line() + 
      theme(axis.text.y = element_blank(), 
            axis.ticks.y = element_blank(), 
            axis.title.y = element_blank(),
            plot.margin = unit(c(0,-2,0,0), "lines"),
            plot.background = element_blank()) +
      ggtitle("norm")

gt1 <- ggplotGrob(p1)
gt2 <- ggplotGrob(p2)

newWidth = unit.pmax(gt1$widths[2:3], gt2$widths[2:3])

gt1$widths[2:3] = as.list(newWidth)
gt2$widths[2:3] = as.list(newWidth)

 # New gtable with space for the three plots plus a right-hand margin
gt = gtable(widths = unit(c(1, 1, 1, .3), "null"), height = unit(1, "null"))

# Instert gt1, gt2 and gt2 into the new gtable
gt <- gtable_add_grob(gt, gt1, 1, 1)
gt <- gtable_add_grob(gt, gt2, 1, 2)
gt <- gtable_add_grob(gt, gt2, 1, 3)

grid.newpage()
grid.draw(gt)

The accepted answer is exactly what makes people run when comes to plotting using R! This is my solution:

library('grid')
g1 <- ggplot(...)  # however you draw your 1st plot
g2 <- ggplot(...)  # however you draw your 2nd plot
grid.newpage()
grid.draw(cbind(ggplotGrob(g1), ggplotGrob(g2), size = "last"))

This takes care of the y axis (minor and major) guide-lines to align in multiple plots, effortlessly.

Dropping some axis text, unifying the legends, ..., are other tasks that can be taken care of while creating the individual plots, or by using other means provided by grid or gridExtra packages.