facet_zoom() while setting axis limits

Question

I would like to use facet_zoom() to zoom in on part of an axis that has limits explicitly set. However, using scale_*(limits = *) and coord_cartesian(xlim = *) overrides the zoomed facet's scales as well such that both have the same limits. Is there a way around this? Maybe I could add some data points near the limits and then set their alpha = 0... Any other ideas?

library(ggplot2)
library(ggforce)

# works with no limits specified
ggplot(mpg, aes(x = hwy, y = cyl)) + 
  geom_point() +
  facet_zoom(xlim = c(20, 25))

# fails with limits specified
ggplot(mpg, aes(x = hwy, y = cyl)) + 
  scale_x_continuous(limits = c(0, 50)) +
  geom_point() +
  facet_zoom(xlim = c(20, 25))

# fails with coord_cartesian()
ggplot(mpg, aes(x = hwy, y = cyl)) + 
  scale_x_continuous() +
  coord_cartesian(xlim = c(0, 50)) +
  geom_point() +
  facet_zoom(xlim = c(20, 25))

Answer 1

I don't have enough knowledge of the underlying intricacies in FacetZoom, but you can check if the following workarounds provide a reasonable starting point.

Plot for demonstration

One of the key differences between setting limits in scales_* vs. coord_* is the clipping effect (screenshot taken from the ggplot2 cheatsheet found here). Since this effect isn't really clear in a scatterplot, I added a geom_line layer and adjusted the specified limits so that the limits extend beyond the data range on one end of the x-axis, & clips the data on the other end.

p <- ggplot(mpg, aes(x = hwy, y = cyl)) +
  geom_point() +
  geom_line(aes(colour = fl), size = 2) +
  facet_zoom(xlim = c(20, 25)) +
  theme_bw()

# normal zoomed plot / zoomed plot with limits set in scale / coord
p0 <- p
p1 <- p + scale_x_continuous(limits = c(0, 35))
p2 <- p + coord_cartesian(xlim = c(0, 35))

We can see that while p0 behaves as expected, both p1 & p2 show both the original facet (top) & the zoomed facet (bottom) with the same range of c(0, 35).

In p1's case, the shaded box also expanded to cover the entire top facet. In p2's case, the zoom box stayed in exactly the same position as p0, & as a result no longer covers the zoomed range of c(20, 25).

Workaround for limits set in scale_*

# convert ggplot objects to form suitable for rendering
gp0 <- ggplot_build(p0)
gp1 <- ggplot_build(p1)

# re-set zoomed facet's limits to match zoomed range
k <- gp1$layout$layout$SCALE_X[gp1$layout$layout$name == "x"]
gp1$layout$panel_scales_x[[k]]$limits <- gp1$layout$panel_scales_x[[k]]$range$range 

# re-set zoomed facet's panel parameters based on original version p0
k <- gp1$layout$layout$PANEL[gp1$layout$layout$name == "x"]
gp1$layout$panel_params[[k]] <- gp0$layout$panel_params[[k]]

# convert built ggplot object to gtable of grobs as usual & print result
gt1 <- ggplot_gtable(gp1)
grid::grid.draw(gt1)

The zoomed facet now shows the zoomed range c(20, 25) correctly, while the shaded box shrinks to cover the correct range in the original facet. Since this method removes unseen data points, all lines in the original facet stay within the confines of the facet.

Workaround for limits set in coord_*

# convert ggplot objects to form suitable for rendering
gp0 <- ggplot_build(p0)
gp1 <- ggplot_build(p1)

# apply coord limits to original facet's scale limits
k <- gp2$layout$layout$SCALE_X[gp2$layout$layout$name == "orig"]
gp2$layout$panel_scales_x[[k]]$limits <- gp2$layout$coord$limits$x

# re-set zoomed facet's panel parameters based on original version without setting
# limits in scale
k <- gp1$layout$layout$PANEL[gp1$layout$layout$name == "x"]
gp2$layout$panel_params[[k]] <- gp0$layout$panel_params[[k]]

# convert built ggplot object to gtable of grobs as usual,
# & print result
gt2 <- ggplot_gtable(gp2)
grid::grid.draw(gt2)

The zoomed facet now shows the zoomed range c(20, 25) correctly, while the shaded box shifts to cover the correct range in the original facet. Since this method includes unseen data points, some lines in the original facet extend beyond the facet's confines.

Note: These workarounds should work with zoom in y + limits set in y-axis as well, as long as all references to "x" / panel_scales_x / SCALE_X above are changed to "y" / panel_scales_y / SCALE_Y. I haven't tested this for other combinations such as zoom in both x & y, but the broad principle ought to be similar.