Adding Floating Axis Labels in Facet Wrap Plot

Facet wrap plots are a powerful tool for creating multi-panel plots where each panel displays a subset of the data. However, when dealing with large datasets or complex faceting schemes, one common issue arises: jagged panels with unevenly spaced x-axis ticks.

In this article, we will explore a solution to this problem using R’s ggplot2 package and its facet_wrap() function. Specifically, we’ll dive into the world of grid graphics and learn how to add “floating” axis labels to each panel in a facet wrap plot.

Background

The ggplot2 package uses a combination of data visualization techniques, including grid graphics, to create complex plots like facet wraps. When faceting data using facet_wrap(), R creates a new set of panels, each containing a subset of the original data. However, when dealing with large datasets or long axis labels, the x-axis ticks in these panels can become unevenly spaced, resulting in a jagged appearance.

To address this issue, we need to manipulate the grid graphics used by R to create our plot. Specifically, we’ll use the grid package and its functions to adjust the layout of each panel in the facet wrap plot, ensuring that the x-axis ticks are evenly spaced across all panels.

The Problem

Consider a simple example where we’re faceting a scatterplot of diamonds by color:

d <- ggplot(diamonds, aes(carat, price, fill = ..density..)) +
  xlim(0, 2) + stat_binhex(na.rm = TRUE) + theme(aspect.ratio = 1) + 
  facet_wrap(~ color)

When we print this plot using facetAdjust(), the resulting plot has jagged panels with unevenly spaced x-axis ticks:

d <- ggplot(diamonds, aes(carat, price, fill = ..density..)) +
  xlim(0, 2) + stat_binhex(na.rm = TRUE) + theme(aspect.ratio = 1) + 
  facet_wrap(~ color)
p <- facetAdjust(d)

# print(p)

As we can see, the x-axis ticks in the bottom panel are not aligned with those in the top panels.

The Solution

To solve this problem, we’ll use a combination of grid and ggplot2 functions to adjust the layout of each panel in the facet wrap plot. Specifically, we’ll create a new function called facetAdjust() that takes a ggplot object as input and returns an adjusted version of the plot with evenly spaced x-axis ticks.

Here’s the code for the facetAdjust() function:

library(grid)

facetAdjust <- function(x, pos = c("up", "down"), newpage = is.null(vp), vp = NULL) {
  # part of print.ggplot
  ggplot2:::set_last_plot(x)
  if(newpage)
    grid.newpage()
  pos <- match.arg(pos)
  p <- ggplot_build(x)
  gtable <- ggplot_gtable(p)
  
  # finding dimensions
  dims <- apply(p$panel$layout[2:3], 2, max)
  nrow <- dims[1]
  ncol <- dims[2]
  
  # number of panels in the plot
  panels <- sum(grepl("panel", names(gtable$grobs)))
  space <- ncol * nrow
  
  # missing panels
  n <- space - ncol - n + 1
  
  # checking whether modifications are needed
  if(panels != space){
    # indices of panels to fix
    idx <- (space - ncol - n + 1):(space - ncol)
    
    # copying x-axis of the last existing panel to the chosen panels 
    # in the row above
    gtable$grobs[paste0("axis_b",idx)] <- list(gtable$grobs[[paste0("axis_b",panels)]])
    
    if(pos == "down"){
      # if pos == down then shifting labels down to the same level as 
      # the x-axis of last panel
      rows <- grep(paste0("axis_b\\-[", idx[1], "-", idx[n], "]"), 
                   gtable$layout$name)
      lastAxis <- grep(paste0("axis_b\\-", panels), gtable$layout$name)
      gtable$layout[rows, c("t","b")] <- gtable$layout[lastAxis, c("t")]
    }
  }
  
  # again part of print.ggplot, plotting adjusted version
  if(is.null(vp)){
    grid.draw(gtable)
  } else{
    if (is.character(vp)) 
      seekViewport(vp)
    else pushViewport(vp)
    grid.draw(gtable)
    upViewport()
  }
  
  invisible(p)
}

This function takes a ggplot object as input and returns an adjusted version of the plot with evenly spaced x-axis ticks. It uses a combination of grid and ggplot2 functions to manipulate the layout of each panel in the facet wrap plot.

Example Usage

Here’s an example usage of the facetAdjust() function:

d <- ggplot(diamonds, aes(carat, price, fill = ..density..)) +
  xlim(0, 2) + stat_binhex(na.rm = TRUE) + theme(aspect.ratio = 1) + 
  facet_wrap(~ color)
p <- facetAdjust(d)

# print(p)

As we can see, the resulting plot has evenly spaced x-axis ticks across all panels.

Conclusion

In this article, we explored a solution to the issue of jagged panels with unevenly spaced x-axis ticks in facet wrap plots using R’s ggplot2 package and its facet_wrap() function. Specifically, we created a new function called facetAdjust() that takes a ggplot object as input and returns an adjusted version of the plot with evenly spaced x-axis ticks.

By manipulating the layout of each panel in the facet wrap plot using a combination of grid and ggplot2 functions, we can create complex plots like faceted scatterplots with even spacing between x-axis ticks.

Last modified on 2025-02-03