ggplot2 dual axes

ggplot2 dual axes

## Dual axes for ggplot2
Modified from:
>https://stackoverflow.com/questions/21026598/ggplot2-adding-secondary-transformed-x-axis-on-top-of-plot
>https://rpubs.com/kohske/dual_axis_in_ggplot2

ggplot_dual_axis.R

ggplot_dual_axis <- function(plot1, plot2, which.axis = "x") {
  
  # Update plot with transparent panel
  plot2 = plot2 + theme(panel.background = element_rect(fill = NA))
  
  grid.newpage()
  
  # Increase right margin if which.axis == "y"
  if(which.axis == "y") plot1 = plot1 + theme(plot.margin = unit(c(0.7, 1.5, 0.4, 0.4), "cm"))
  
  # Extract gtable
  g1 = ggplot_gtable(ggplot_build(plot1))
  
  g2 = ggplot_gtable(ggplot_build(plot2))
  
  # Overlap the panel of the second plot on that of the first
  pp = c(subset(g1$layout, name == "panel", se = t:r))
  
  g = gtable_add_grob(g1, g2$grobs[[which(g2$layout$name=="panel")]], pp$t, pp$l, pp$b, pp$l)

  # Steal axis from second plot and modify
  axis.lab = ifelse(which.axis == "x", "axis-b", "axis-l")
  
  ia = which(g2$layout$name == axis.lab)
  
  ga = g2$grobs[[ia]]
  
  ax = ga$children[[2]]
  
  # Switch position of ticks and labels
  if(which.axis == "x") ax$heights = rev(ax$heights) else ax$widths = rev(ax$widths)
  
  ax$grobs = rev(ax$grobs)
  
  if(which.axis == "x") 
    
    ax$grobs[[2]]$y = ax$grobs[[2]]$y - unit(1, "npc") + unit(0.15, "cm") else
      
      ax$grobs[[1]]$x = ax$grobs[[1]]$x - unit(1, "npc") + unit(0.15, "cm")
  
  # Modify existing row to be tall enough for axis
  if(which.axis == "x") g$heights[[2]] = g$heights[g2$layout[ia,]$t]
  
  # Add new row or column for axis label
  if(which.axis == "x") {
    
    g = gtable_add_grob(g, ax, 2, 4, 2, 4) 
    
    g = gtable_add_rows(g, g2$heights[1], 1)
        
  } else {
    
    g = gtable_add_cols(g, g2$widths[g2$layout[ia, ]$l], length(g$widths) - 1)
    
    g = gtable_add_grob(g, ax, pp$t, length(g$widths) - 1, pp$b) 
        
  }
  
  # Draw it
  grid.draw(g)
  
}