Superimpose one ggplot on another

Edit after extra requirements You may use cowplot::ggdraw to superimpose the plot2-grob over plot1 positioning it within plot 1 using normalized device coordinates [0,1] + setting the background to semi-transparent using panel.background = elementrect(fill = alpha('#D3D3D3', 0.6)

• plot margins can be defined within the theme of plot2 using this
• if you want to set more plot elements to transparent, check this out (e.g. hiding the grids panel.grid = elementblank())

library(ggplot2)
library(cowplot)
set.seed(1)
dat1 = data.frame(x = rnorm(1000), y = rnorm(1000))
dat2 = data.frame(x = rt(5000, 2))
Plot1 = ggplot(data = dat1, aes(x = x, y = y)) + geompoint() +
  theme(plot.background = elementrect(fill='transparent', color="black"))
Plot2 = ggplot(dat2) + geomhistogram(aes(x = x)) + 
  theme(
    # add half-transparent background
    panel.background = elementrect(fill = alpha('#D3D3D3', 0.5)), 
    #                                       add border color and      linewidth if needed
    plot.background = elementrect(fill='transparent', color="black", linewidth=1)
  )
ggdraw() +
  drawplot(Plot1) +
  draw_plot(Plot2, x = 0.05, y = 0.3, width = 0.4, height = 0.4)
Adjust x,y,width,height as relatives of plot 1 dimensions
here i set the width of Plot 2 to be 40 % of Plot 1's width

giving