How to change the position of the zoomed area from facet_zoom()?

Question

With facet_zoom() from the ggforce package one can create nice zooms to highlight certain regions of a plot. Unfortunately, when zooming in on the y axis the original plot is always on the right side.

Is there a way to place the original plot on the left?

This would feel more intuitive to first look at the main plot and then at the zoomed region. As an example I would like to swap the position of the two facets in this plot:

(No reproducible example added, since I believe this is a question about the existence of a certain functionality.)

Answer 1

I've tweaked the current code for FacetZoom on GitHub to swop the horizontal order from [zoom, original] to [original, zoom]. The changes aren't complicated, but they are scattered throughout draw_panels() function's code, so the full code is rather long.

Result:

# example 1, with split = FALSE, horizontal = TRUE (i.e. default settings)
p1 <- ggplot(mtcars, aes(x = mpg, y = disp, colour = factor(cyl))) +
  geom_point() +
  theme_bw()    
p1 + ggtitle("Original") + facet_zoom(y = disp > 300)
p1 + ggtitle("Modified") + facet_zoom2(y = disp > 300)

# example 2, with split = TRUE
p2 <- ggplot(iris, aes(Petal.Length, Petal.Width, colour = Species)) +
  geom_point() +
  theme_bw()    
p2 + ggtitle("Original") + 
  facet_zoom(xy = Species == "versicolor", split = TRUE)
p2 + ggtitle("Modified") + 
  facet_zoom2(xy = Species == "versicolor", split = TRUE)

Code used (I've commented out the original code, where modified code is used, & indicated the packages for functions from other packages):

library(ggplot)
library(ggforce)
library(grid)

# define facet_zoom2 function to use FacetZoom2 instead of FacetZoom
# (everything else is the same as facet_zoom)
facet_zoom2 <- function(x, y, xy, zoom.data, xlim = NULL, ylim = NULL, 
                        split = FALSE, horizontal = TRUE, zoom.size = 2, 
                        show.area = TRUE, shrink = TRUE) {
  x <- if (missing(x)) if (missing(xy)) NULL else lazyeval::lazy(xy) else lazyeval::lazy(x)
  y <- if (missing(y)) if (missing(xy)) NULL else lazyeval::lazy(xy) else lazyeval::lazy(y)
  zoom.data <- if (missing(zoom.data)) NULL else lazyeval::lazy(zoom.data)
  if (is.null(x) && is.null(y) && is.null(xlim) && is.null(ylim)) {
    stop("Either x- or y-zoom must be given", call. = FALSE)
  }
  if (!is.null(xlim)) x <- NULL
  if (!is.null(ylim)) y <- NULL
  ggproto(NULL, FacetZoom2,
          shrink = shrink,
          params = list(
            x = x, y = y, xlim = xlim, ylim = ylim, split = split, zoom.data = zoom.data,
            zoom.size = zoom.size, show.area = show.area,
            horizontal = horizontal
          )
  )
}

# define FacetZoom as a ggproto object that inherits from FacetZoom,
# with a modified draw_panels function. the compute_layout function references
# the version currently on GH, which is slightly different from the CRAN
# package version.
FacetZoom2 <- ggproto(
  "FacetZoom2",
  ggforce::FacetZoom,

  compute_layout = function(data, params) {
    layout <- rbind( # has both x & y dimension
      data.frame(name = 'orig', SCALE_X = 1L, SCALE_Y = 1L),
      data.frame(name = 'x', SCALE_X = 2L, SCALE_Y = 1L),
      data.frame(name = 'y', SCALE_X = 1L, SCALE_Y = 2L),
      data.frame(name = 'full', SCALE_X = 2L, SCALE_Y = 2L),
      data.frame(name = 'orig_true', SCALE_X = 1L, SCALE_Y = 1L),
      data.frame(name = 'zoom_true', SCALE_X = 1L, SCALE_Y = 1L)
    )
    if (is.null(params$y) && is.null(params$ylim)) { # no y dimension
      layout <- layout[c(1,2, 5:6),]
    } else if (is.null(params$x) && is.null(params$xlim)) { # no x dimension
      layout <- layout[c(1,3, 5:6),]
    }
    layout$PANEL <- seq_len(nrow(layout))
    layout
  },

  draw_panels = function(panels, layout, x_scales, y_scales, ranges, coord,
                         data, theme, params) {

    if (is.null(params$x) && is.null(params$xlim)) {
      params$horizontal <- TRUE
    } else if (is.null(params$y) && is.null(params$ylim)) {
      params$horizontal <- FALSE
    }
    if (is.null(theme[['zoom']])) {
      theme$zoom <- theme$strip.background
    }
    if (is.null(theme$zoom.x)) {
      theme$zoom.x <- theme$zoom
    }
    if (is.null(theme$zoom.y)) {
      theme$zoom.y <- theme$zoom
    }
    axes <- render_axes(ranges, ranges, coord, theme, FALSE)
    panelGrobs <- ggforce:::create_panels(panels, axes$x, axes$y)
    panelGrobs <- panelGrobs[seq_len(length(panelGrobs) - 2)]
    if ('full' %in% layout$name && !params$split) {
      panelGrobs <- panelGrobs[c(1, 4)]
    }

    # changed coordinates in indicator / lines to zoom from 
    # the opposite horizontal direction
    if ('y' %in% layout$name) {
      if (!inherits(theme$zoom.y, 'element_blank')) {
        zoom_prop <- scales::rescale(
          y_scales[[2]]$dimension(ggforce:::expansion(y_scales[[2]])),
          from = y_scales[[1]]$dimension(ggforce:::expansion(y_scales[[1]])))
        indicator <- polygonGrob(
          x = c(0, 0, 1, 1), # was x = c(1, 1, 0, 0), 
          y = c(zoom_prop, 1, 0), 
          gp = gpar(col = NA, fill = alpha(theme$zoom.y$fill, 0.5)))
        lines <- segmentsGrob(
          x0 = c(1, 1), x1 = c(0, 0), # was x0 = c(0, 0), x1 = c(1, 1)
          y0 = c(0, 1), y1 = zoom_prop,
          gp = gpar(col = theme$zoom.y$colour,
                    lty = theme$zoom.y$linetype,
                    lwd = theme$zoom.y$size,
                    lineend = 'round'))
        indicator_h <- grobTree(indicator, lines)
      } else {
        indicator_h <- zeroGrob()
      }
    }

    if ('x' %in% layout$name) {
      if (!inherits(theme$zoom.x, 'element_blank')) {
        zoom_prop <- scales::rescale(x_scales[[2]]$dimension(ggforce:::expansion(x_scales[[2]])),
                                     from = x_scales[[1]]$dimension(ggforce:::expansion(x_scales[[1]])))
        indicator <- polygonGrob(c(zoom_prop, 1, 0), c(1, 1, 0, 0), 
                                 gp = gpar(col = NA, fill = alpha(theme$zoom.x$fill, 0.5)))
        lines <- segmentsGrob(x0 = c(0, 1), y0 = c(0, 0), x1 = zoom_prop, y1 = c(1, 1), 
                              gp = gpar(col = theme$zoom.x$colour,
                                        lty = theme$zoom.x$linetype,
                                        lwd = theme$zoom.x$size,
                                        lineend = 'round'))
        indicator_v <- grobTree(indicator, lines)
      } else {
        indicator_v <- zeroGrob()
      }
    }

    if ('full' %in% layout$name && params$split) {
      space.x <- theme$panel.spacing.x
      if (is.null(space.x)) space.x <- theme$panel.spacing
      space.x <- unit(5 * as.numeric(convertUnit(space.x, 'cm')), 'cm')
      space.y <- theme$panel.spacing.y
      if (is.null(space.y)) space.y <- theme$panel.spacing
      space.y <- unit(5 * as.numeric(convertUnit(space.y, 'cm')), 'cm')

      # change horizontal order of panels from [zoom, original] to [original, zoom]
      # final <- gtable::gtable_add_cols(panelGrobs[[3]], space.x)
      # final <- cbind(final, panelGrobs[[1]], size = 'first')
      # final_tmp <- gtable::gtable_add_cols(panelGrobs[[4]], space.x)
      # final_tmp <- cbind(final_tmp, panelGrobs[[2]], size = 'first')
      final <- gtable::gtable_add_cols(panelGrobs[[1]], space.x)
      final <- cbind(final, panelGrobs[[3]], size = 'first')
      final_tmp <- gtable::gtable_add_cols(panelGrobs[[2]], space.x)
      final_tmp <- cbind(final_tmp, panelGrobs[[4]], size = 'first')

      final <- gtable::gtable_add_rows(final, space.y)
      final <- rbind(final, final_tmp, size = 'first')
      final <- gtable::gtable_add_grob(final, list(indicator_h, indicator_h),
                                       c(2, 6), 3, c(2, 6), 5,
                                       z = -Inf, name = "zoom-indicator")
      final <- gtable::gtable_add_grob(final, list(indicator_v, indicator_v), 
                                       3, c(2, 6), 5, 
                                       z = -Inf, name = "zoom-indicator")
      heights <- unit.c(
        unit(max_height(list(axes$x[[1]]$top, axes$x[[3]]$top)), 'cm'),
        unit(1, 'null'),
        unit(max_height(list(axes$x[[1]]$bottom, axes$x[[3]]$bottom)), 'cm'),
        space.y,
        unit(max_height(list(axes$x[[2]]$top, axes$x[[4]]$top)), 'cm'),
        unit(params$zoom.size, 'null'),
        unit(max_height(list(axes$x[[2]]$bottom, axes$x[[4]]$bottom)), 'cm')
      )

      # swop panel width specifications according to the new horizontal order
      widths <- unit.c(
        # unit(max_width(list(axes$y[[3]]$left, axes$y[[4]]$left)), 'cm'),
        # unit(params$zoom.size, 'null'),
        # unit(max_height(list(axes$y[[3]]$right, axes$y[[4]]$right)), 'cm'),
        # space.x,
        # unit(max_width(list(axes$y[[1]]$left, axes$y[[2]]$left)), 'cm'),
        # unit(1, 'null'),
        # unit(max_height(list(axes$y[[1]]$right, axes$y[[2]]$right)), 'cm')        
        unit(max_width(list(axes$y[[1]]$left, axes$y[[2]]$left)), 'cm'),
        unit(1, 'null'),
        unit(max_height(list(axes$y[[1]]$right, axes$y[[2]]$right)), 'cm'),
        space.x,
        unit(max_width(list(axes$y[[3]]$left, axes$y[[4]]$left)), 'cm'),
        unit(params$zoom.size, 'null'),
        unit(max_height(list(axes$y[[3]]$right, axes$y[[4]]$right)), 'cm')

      )
      final$heights <- heights
      final$widths <- widths
    } else {
      if (params$horizontal) {
        space <- theme$panel.spacing.x
        if (is.null(space)) space <- theme$panel.spacing
        space <- unit(5 * as.numeric(convertUnit(space, 'cm')), 'cm')
        heights <- unit.c(
          unit(max_height(list(axes$x[[1]]$top, axes$x[[2]]$top)), 'cm'),
          unit(1, 'null'),
          unit(max_height(list(axes$x[[1]]$bottom, axes$x[[2]]$bottom)), 'cm')
        )

        # change horizontal order of panels from [zoom, original] to [original, zoom]
        # first <- gtable::gtable_add_cols(panelGrobs[[2]], space)
        # first <- cbind(final, panelGrobs[[1]], size = 'first')
        final <- gtable::gtable_add_cols(panelGrobs[[1]], space) 
        final <- cbind(final, panelGrobs[[2]], size = "first") 

        final$heights <- heights

        # swop panel width specifications according to the new horizontal order
        # unit(c(params$zoom.size, 1), 'null')
        final$widths[panel_cols(final)$l] <- unit(c(1, params$zoom.size), 'null') 

        final <- gtable::gtable_add_grob(final, indicator_h, 2, 3, 2, 5, 
                                         z = -Inf, name = "zoom-indicator")
      } else {
        space <- theme$panel.spacing.y
        if (is.null(space)) space <- theme$panel.spacing
        space <- unit(5 * as.numeric(convertUnit(space, 'cm')), 'cm')
        widths <- unit.c(
          unit(max_width(list(axes$y[[1]]$left, axes$y[[2]]$left)), 'cm'),
          unit(1, 'null'),
          unit(max_height(list(axes$y[[1]]$right, axes$y[[2]]$right)), 'cm')
        )
        final <- gtable::gtable_add_rows(panelGrobs[[1]], space)
        final <- rbind(final, panelGrobs[[2]], size = 'first')
        final$widths <- widths
        final$heights[panel_rows(final)$t] <- unit(c(1, params$zoom.size), 'null')
        final <- gtable::gtable_add_grob(final, indicator_v, 3, 2, 5, 
                                         z = -Inf, name = "zoom-indicator")
      }
    }
    final
  }
)

Note: create_panels and expansion are un-exported functions from the ggforce package, so I referenced them with triple colons. This isn't robust for writing packages, but should suffice as a temporary workaround.

Update 30 Oct 2019: A suggestion for those seeing errors like Invalid 'type' (list) of argument after trying to use this solution as-is. The issue is likely due to updates made to the ggforcepackage since this solution was developed. To get the code in this solution working again, install the version of ggforce that was available when the solution was developed. This can be done with the devtools package pointing to the 4008a2e commit:

devtools::install_github("thomasp85/ggforce", ref = '4008a2e')