How to apply a clipping mask to geom in a ggplot?

Question

I am trying to apply a clipping mask to a geom from a ggplot to mask part of the data, but keep the axis, the grid, other geoms and the legend visible. I do not want to create a specific plot, and therefore I am not looking for a work-around with polygons masking some parts of the plot.

This is the kind of design I would like to emulate (the mask, not necessarily theme, I now how to do that):

(source)

See also this example

One could argue that I could filter the data that is not contained in the polygon that defines the mask. But, while it works for point, and can work for polygon/line-like objects, it does for rasters (the border would not exactly follow non-vertical or non-horizontal lines). So I tried the following:

library(ggplot2)
library(gridSVG)
library(grImport)

# Create a plot
p <- ggplot(diamonds[1:300,], aes(carat, price)) + geom_point(aes(colour = cut))

# And a clipping mask
pg <- polygonGrob(c(.7, 0, 0, 1, 1),
              c(0, .7, 1, 1, 0))
cp <- clipPath(pg)

I was able to use the packages gridSVG to define a clipping mask, but I have difficulties applying it on a ggplot object, even after extracting the grob (see resource here) with ggplotGrob(). I was not able to apply the clipping mask to the grob:

g <-  ggplotGrob(p) # store the plot as a grob

registerClipPath("mask", cp)
g_clipped <- clipPath(g)

gridsvg(name = "test_c2.svg")
grid.draw(clipPathGrob(g_clipped, cp)$grob)
dev.off()

My intuition was that the g_clipped should be plotted, but I couldn't grid.draw() it, since its a clipPath object. And the grid.draw() line written here show the plot not masked. I think I don't quite grasp how the clipPath objects function.

The function grobify() sounds like it could help for an alternative appraoch without gridSVG, see details here, but I do not understand the quite minimalistic documentation.

Since I cannot even apply the clipping mask to the whole plot, I am far for my objective.

If you can help me understand how to apply the clipping mask from gridSVGor have an alternative solution to apply a clipping mask to specific geoms, please let me know.

Answer 1

The following is a grid solution, but very much a work-around. It shows how to apply a non-rectangular clipping region to a ggplot, so that one set of points in your plot is clipped. You weren't too far wrong in your attempt. A couple of points to note:

1. You need to grid.force() the ggplotGrob object so the grid can see the grobs.
2. Do not define the ggplot grob as a clipping path - the clipping path is the polygon.
3. The clipping path is applied to the points grob within the plot panel of the ggplot. This means that other objects in the plot panel, the panel background and grid lines, do not get clipped. Only the data points are clipped.

I've added a blue line to the plot to show that the line too does not need to be clipped; but can be clipped if desired.

There are also commented lines of code that, when uncommented, will draw the clipping region, and move the grid lines and points to the front (that is, in front of the darker grey clipping region).

library(ggplot2)
library(gridSVG)
library(grid)

# Open the graphics device
gridsvg(name = "test.svg")

# Create a plot
p <- ggplot(diamonds[1:300, ], aes(carat, price)) + 
       geom_point(aes(colour = cut)) +
       geom_line(data = data.frame(x = c(.3, .9), y = c(500, 2500)), aes(x,y), col = "skyblue", size = 2)
g <- ggplotGrob(p) # Store the plot as a grob


g = grid.force(g)  # So that grid sees all grobs
grid.draw(g)       # Draw the plot

# Define the clipping path
pg <- polygonGrob(c(.7, 0, 0, 1, 1),
                  c(0, .7, 1, 1, 0))
# The clipping path can be nearly any shape you desire. 
# Try this for a circular region
# pg = circleGrob(x = .5, y = .6, r = .5)
cp <- clipPath(pg)

# Add the clipping path to the points grob.
# That is, only the points inside the polygon will be visible,
# but the background and grid lines will not be clipped. 
# Nor will the blue line be clipped.
# grid.ls(g)     # names of the grobs
seekViewport(grep("panel.[0-9]", grid.ls(g)$name, value = TRUE))
grid.clipPath("points", cp, grep = TRUE)   

# To clip the blue line, uncomment the next line
# grid.clipPath("GRID.polyline", cp, grep = TRUE)       

# To show the clipping region,    
# uncomment the next two lines.
# showcp = editGrob(pg, gp = gpar(fill = rgb(0, 0, 0, 0.05), col = "transparent"))
# grid.draw(showcp)

# And to move the grid lines, remaining data points, and blue line in front of the clipping region,
# uncomment the next five lines
# panel = grid.get("panel", grep = TRUE)   # Get the panel, and remove the background grob
# panel = removeGrob(panel, "background", grep = TRUE)

# grid.remove("points", grep = TRUE)     # Remove points and grid lines from the rendered plot
# grid.remove("line", grep = TRUE, global = TRUE)

# grid.draw(panel)     # Draw the edited panel - on top of the clipping region 


# Turn off the graphics device
dev.off()

# Find text.svg in your working directory

Edit Defining the clipping region using the coordinate system in which the data points were drawn.

library(ggplot2)
library(gridSVG)
library(grid)

# Open the graphics device
gridsvg(name = "test.svg")

# Create a plot
p <- ggplot(diamonds[1:300, ], aes(carat, price)) + 
       geom_point(aes(colour = cut)) +
       geom_line(data = data.frame(x = c(.3, .9), y = c(500, 2500)), aes(x,y), col = "skyblue", size = 2)
g <- ggplotGrob(p) # Store the plot as a grob


g = grid.force(g)  # So that grid sees all grobs
grid.draw(g)       # Draw the plot

# Get axis limits (including any expansion)
axis.limits = summarise_layout(ggplot_build(p))[1, c('xmin', 'xmax', 'ymin', 'ymax')]

# Find the 'panel' viewport,
# then push to a new viewport, 
# one that exactly overlaps the 'panel' viewport,
# but with limits on the x and y scales that are the same
# as the limits for the original ggplot. 
seekViewport(grep("panel.[0-9]", grid.ls(g)$name, value = TRUE))
pushViewport(dataViewport(xscale = axis.limits[1, 1:2],
                          yscale = axis.limits[1, 3:4]))

# Define the clipping path
 pg <- polygonGrob(x = c(.6,   0.3, .3,   .8,   1.2), 
                   y = c(500, 1500, 2900, 2900, 1500), 
                   default.units="native")
cp <- clipPath(pg)

# Add the clipping path to the points grob.
# That is, only the points inside the polygon will be visible,
# but the background and grid lines will not be clipped. 
# Nor will the blue line be clipped.
# grid.ls(g)     # names of the grobs

grid.clipPath("points", cp, grep = TRUE)   

# To clip the blue line, uncomment the next line
 grid.clipPath("GRID.polyline", cp, grep = TRUE)       

# To show the clipping region. 
 showcp = editGrob(pg, gp = gpar(fill = rgb(0, 0, 0, 0.05), col = "transparent"))
 grid.draw(showcp)

# And to move the grid lines and remaining data points in front of the clipping region.
 panel = grid.get("panel", grep = TRUE)   # Get the panel, and remove the background grob
 panel = removeGrob(panel, "background", grep = TRUE)

 grid.remove("points", grep = TRUE)     # Remove points and grid lines from the rendered plot
 grid.remove("line", grep = TRUE, global = TRUE)

 grid.draw(panel)     # Draw the edited panel - on top of the clipping region 


# Turn off the graphics device
dev.off()

# Find text.svg in your working directory