Ggplot and grid: find the relative x and y positions of a point in ggplot grob

I am piling multiple ggplot plots using gridviews, a necessity (I believe) because I want to rotate the plot, which is not possible in standard ggplot and maybe even in the gridExtra package.

I want to draw a line on two plots to make the correlation more clear. But to know exactly where the lines are, I need the relative positions of the point in the ggplot (grob?) Plot.

I made the following example:

require(reshape2)
require(grid)
require(ggplot2)


datamat <- matrix(rnorm(50), ncol=5)
cov_mat <- cov(datamat)
cov_mat[lower.tri(cov_mat)] <- NA

data_df <- melt(datamat)
cov_df <- melt(cov_mat)

plot_1 <- ggplot(data_df, aes(x=as.factor(Var2), y=value)) + geom_boxplot()
plot_2 <- ggplot(cov_df, aes(x=Var1, y=Var2, fill=value)) + 
                    geom_tile() +
                    scale_fill_gradient(na.value="transparent") + 
                    coord_fixed() +
                    theme(
                    legend.position="none",
                    plot.background = element_rect(fill = "transparent",colour = NA),
                    panel.grid=element_blank(),
                    panel.background=element_blank(),
                    panel.border = element_blank(),
                    plot.margin = unit(c(0, 0, 0, 0), "npc"),
                    axis.ticks=element_blank(), 
                    axis.title=element_blank(), 
                    axis.text=element_text(size=unit(0,"npc")),
                    )

cov_heatmap <- ggplotGrob(plot_2)
boxplot <- ggplotGrob(plot_1)

grid.newpage()

pushViewport(viewport(height=unit(sqrt(2* 0.4 ^2), 'npc'),
                      width=unit(sqrt(2* 0.4 ^2), 'npc'),
                      x=unit(0.5, 'npc'),
                      y=unit(0.63, 'npc'),
                      angle=-45,
                      clip="on")
            )
grid.draw(cov_heatmap)
upViewport(0)
pushViewport(viewport(height=unit(0.5, 'npc'),
                      width=unit(1, 'npc'),
                      x=unit(0.5, 'npc'),
                      y=unit(0.25, 'npc'),
                      clip="on")
            )
grid.draw(boxplot)

      

        
        
        
      

    

which creates a graph

How do I find the relative x and y positions (let's say) of the first boxplot? and also the relative x and y positions of the triangular covariance matrix.

I know I need to examine grob objects boxplot

, but I don't know how to find the relevant data there.

EDIT:

I was asked to give an example of a plot with manually added lines like below:

The lines come from points in the lower plot to blocks in the upper plot.