How do you compare the same plot across groups? A facet splits one plot into small multiples: one panel per category of a column. Wilkinson calls facets frames of frames: the facet column’s categories form an outer frame, and each of its cells holds a copy of the inner x/y frame. Nothing else about the plot changes. The marks, channels, and transforms you have already learned draw each panel exactly as they would draw the whole.
The picture has been invented more than once, so you may know it under another name. Becker and Cleveland’s group at Bell Labs built it into S as Trellis display (Becker et al., 1996), the name borrowed from the garden trellis its rows and columns resemble, and S-PLUS is where most people met it. R draws it that way still in the lattice package (Sarkar, 2026), and its own book is where the display is set out in full (Sarkar, 2008). Base R’s coplot() is the same idea under a third name, the conditioning plot, because what a trellis conditions on is the facet column. That is also why the operator in the next section is a bar, a lineage Composition finishes telling. Tufte’s small multiples (Tufte, 1990) names the result rather than the operation, which is the division this book keeps: small multiples for the picture, facet for the thing you write. And facet is Wilkinson’s own word (Wilkinson, 2005), from the Latin facies, a face: one side of a many-sided thing, a cut diamond’s. gog takes his because his is the widest of the four. A trellis, to him, is one arrangement a facet can take, next to the scatterplot matrix and the row plot, and a grammar needs the term that covers all three. The word every one of these systems shares is panel, the cell a split produces, and this chapter uses it the same way.
|: panels side by sideThe facet() atom names the column that does the splitting, and | arranges the panels in columns:
data(gapminder_2007) + point + x(gdp) + y(life) | facet(continent)data(gapminder_2007) + point + x(col.gdp) + y(col.life) | facet(col.continent)data(gapminder_2007) + point + x(:gdp) + y(:life) | facet(:continent)plot(data(gapminder_2007), point, x(col.gdp), y(col.life),
across(col.continent))“Given gapminder 2007: points, x is gdp, y is life, split into panel columns by continent.”
Reading across, the panels share both axes, so a shift between continents reads as plainly as a pattern within one. That shared frame is the point of faceting: the alternative (one plot per continent, each with its own scale) looks similar and quietly stops the comparison working.
/: panels stacked/ arranges the panels in rows. Stacking aligns the x axis, which is the better direction when the comparison is where things sit along x:
gm_ae <- gapminder_2007[gapminder_2007$continent %in% c("Africa", "Americas", "Europe"), ]
data(gm_ae) + bar * bin + x(life) / facet(continent)The three distributions line up column for column: Africa’s life expectancies spread wide and low, Europe’s cluster high and narrow.
That reading only works because the bars are the same width in all three panels, and a histogram under a facet is where the two halves of faceting come apart. Each panel counts its own rows, because a statistic always sees the panel’s subset as if it were the whole data: the No Exceptions law applied to frames. But bin is not only a statistic. It cuts the axis into cells and tallies what lands in them, and the cut is a description of the axis rather than a measurement of the rows, so it belongs with the shared scale. One set of bins is cut from all three panels’ rows at once, and each panel then counts only its own into them.
The alternative is worth picturing. Cut per panel, Europe’s narrow spread earns narrow bins and Africa’s wide one earns wide bins, so bars of different widths stand against a single axis. A bar four years wide holding nine countries and a bar one year wide holding eight then draw at nearly the same height while describing populations twice as different. The panels stay aligned, the axis stays honest, and the comparison the chapter opened with quietly stops working.
Write both operators to cross two columns into a grid. Each operator applies to the facet written after it, read left to right:
gm_years <- gm_all[gm_all$year %in% c(1957, 2007), ]
gm_years$period <- as.character(gm_years$year)
data(gm_years) + point + x(gdp) + y(life) | facet(continent) / facet(period)Fifty years, five continents, one frame: every panel column is a continent, every panel row a period, and the shared scale lets the eye measure the half-century’s climb in life expectancy panel by panel.
The grid is a crossing in Wilkinson’s sense: every row × column combination gets a panel, and a combination with no rows still gets an empty one. The frame says the combination is possible: an empty panel is information, not an accident. (His nesting, draw only the combinations that exist, is a different operation, and these operators deliberately do not claim it.)
One column with many categories makes a long line of panels, and a long line of panels makes each one narrow. Here are the ten most populous countries of 2007, each with its own half-century of life expectancy:
ranked <- gapminder_2007[base::order(-gapminder_2007$population), ]
gm_top <- gm_all[gm_all$country %in% utils::head(ranked$country, 10), ]
data(gm_top) + line + group(country) + x(year) + y(life) | facet(country)The shapes are still there, but nothing else is. Read the axis: the tick labels have run into each other and print as 19502000, and the last strip has more name than panel to write it in. Ten panels is not an unusual number of categories, and it is already past what one line of them can carry.
wrap folds that line into a rectangle. The number is how many panels to draw before the line turns:
data(gm_top) + line + group(country) + x(year) + y(life) | facet(country, wrap = 4)data(gm_top) + line + group(col.country) + x(col.year) + y(col.life) | facet(col.country, wrap = 4)data(gm_top) + line + group(:country) + x(:year) + y(:life) |
facet(:country, wrap = 4)plot(data(gm_top), line, group(col.country), x(col.year), y(col.life),
across(col.country, { wrap: 4 }))Same ten panels, same shared scale, same sentence with one setting added. The years are readable, United States fits its strip, and the climb in each country is worth looking at rather than merely present.
Notice what wrap does not say: which way the panels run. That was settled before wrap was written, by the operator. | runs the line across, so four to a row; / runs it down, so four to a column:
data(gm_top) + line + group(country) + x(year) + y(life) / facet(country, wrap = 4)data(gm_top) + line + group(col.country) + x(col.year) + y(col.life) / facet(col.country, wrap = 4)data(gm_top) + line + group(:country) + x(:year) + y(:life) /
facet(:country, wrap = 4)plot(data(gm_top), line, group(col.country), x(col.year), y(col.life),
down(col.country, { wrap: 4 }))The same number, the same ten countries, read down the columns instead of across the rows. One number is enough because the direction is already in the sentence, which is why there is no second setting for it. ggplot2 spells the same idea as nrow and ncol.
Two details in the wrapped plots are the fold showing through. The first is the bottom right, where the last row stops after two panels: those cells are the slack that folding ten panels into a twelve-cell rectangle leaves over, so nothing is drawn in them. They are not the empty panels of the previous section. An empty panel in a crossing is a combination the frame says is possible and the data has no example of, which is information; here there is no combination at all, only room.
The second is the axis under Mexico and Nigeria. Tick labels belong to panels that touch the margin the axis lives in, which in a full grid is the bottom row. Once a row is ragged that stops being the same sentence, so the rule is the more general one it was always a special case of: a panel draws the x axis when no panel sits below it. Mexico and Nigeria hang over the gap, so they keep their years.
Wrapping a crossed grid is refused. The crossing has already made a rectangle out of two columns, and a count beside it could only be a second opinion about the same shape:
render_svg(data(gm_years) + point + x(gdp) + y(life) |
facet(continent, wrap = 2) / facet(period))Error:
! gog: `wrap` folds one line of panels into a rectangle, but `continent` and `period` already cross into one. Drop `wrap`, or facet by one column and let `wrap` shape it.
gog: nothing was rendered. Fix the above, or set GOG_STRICT=0 to draw anyway.Everything so far has rested on one scale across every panel, which is what makes the panels comparable. It is also what makes some of them unreadable. Here is the world’s population by continent, one panel each:
data(gm_all) + line * sum + x(year) + y(population) | facet(continent, wrap = 3)data(gm_all) + line * sum + x(col.year) + y(col.population) | facet(col.continent, wrap = 3)data(gm_all) + line * sum + x(:year) + y(:population) |
facet(:continent, wrap = 3)plot(data(gm_all), layer(line, sum), x(col.year), y(col.population),
across(col.continent, { wrap: 3 }))Asia is four billion people and Oceania is twenty million, so the axis is spent on Asia and Oceania is a flat line along the bottom. Its shape is in the picture and cannot be seen. The plot is not wrong: it is telling you, correctly and loudly, that these five quantities are not the same size. But if the question was “how did each continent grow”, it has answered a different one.
free = TRUE on the position fits that axis from each panel’s own rows:
data(gm_all) + line * sum + x(year) +
y(population, free = TRUE) | facet(continent, wrap = 3)(data(gm_all) + line * sum + x(col.year) +
y(col.population, free = True) | facet(col.continent, wrap = 3))data(gm_all) + line * sum + x(:year) + y(:population, free = true) |
facet(:continent, wrap = 3)plot(data(gm_all), layer(line, sum), x(col.year),
y(col.population, { free: true }), across(col.continent, { wrap: 3 }))Now every panel has a shape, and Oceania turns out to have more than doubled. Notice the y axis: each panel carries its own numbers, because one set of labels along the edge was only ever enough while one scale was.
Notice also what the second plot costs, because it is the reason this is a request and never a default. All five panels now look alike: five lines rising left to right at about the same angle. Reading them side by side, nothing says that one of those climbs is measured in billions and another in tens of millions, and a reader who glances at the shapes without reading the axes will take away something false. The first plot cannot mislead that way and the second can, which is why gog will not do this unless you ask.
Which axis is freed is decided by where you write it. That is the same trick wrap plays with | and /: the sentence already names the axis, so free does not name it again. There is no free_x or free_y, and freeing both is writing it twice:
data(gm_all) + point + x(gdp, free = TRUE) + y(life, free = TRUE) |
facet(continent, wrap = 3)(data(gm_all) + point + x(col.gdp, free = True) + y(col.life, free = True) |
facet(col.continent, wrap = 3))data(gm_all) + point + x(:gdp, free = true) + y(:life, free = true) |
facet(:continent, wrap = 3)plot(data(gm_all), point, x(col.gdp, { free: true }),
y(col.life, { free: true }), across(col.continent, { wrap: 3 }))There is a third position, and it takes free on the same terms. Panels of cubes are panels:
data(gapminder_2007) + point + x(gdp) + y(life) +
z(population, free = TRUE) + color(continent) | facet(continent)(data(gapminder_2007) + point + x(col.gdp) + y(col.life) +
z(col.population, free = True) + color(col.continent) | facet(col.continent))data(gapminder_2007) + point + x(:gdp) + y(:life) +
z(:population, free = true) + color(:continent) | facet(:continent)plot(data(gapminder_2007), point, x(col.gdp), y(col.life),
z(col.population, { free: true }), color(col.continent),
across(col.continent))Shared, the height axis has to reach China, so Oceania’s two countries lie flat on the floor with nothing to read. Freed, Oceania is measured in tens of millions and has a shape again, at the price this whole section is about: the five cubes no longer compare. Nothing was added to the grammar to reach that plot. free is written on a position and z is a position, so the third one has always accepted it, and the sentence only became possible to write when the cube learned to sit in a panel.
Three things travel with a free scale, and none of them is separately requestable. The first is the bin cut. The histogram earlier in this chapter shares one set of bin edges across every panel, because a cut describes the axis and the axis was shared. Free the axis and the edges follow it, for exactly the same reason:
data(gm_all) + bar * bin + x(gdp, free = TRUE) | facet(continent, wrap = 3)data(gm_all) + bar * bin + x(col.gdp, free = True) | facet(col.continent, wrap = 3)data(gm_all) + bar * bin + x(:gdp, free = true) |
facet(:continent, wrap = 3)plot(data(gm_all), layer(bar, bin), x(col.gdp, { free: true }),
across(col.continent, { wrap: 3 }))Each panel is cut from its own rows, so Africa gets bins the size of Africa’s spread rather than bins sized for Norway. There is no way to ask for a free scale over a shared cut, which would be a picture whose bars are the panel’s and whose edges are the plot’s.
The second is the panel’s guides. A scale says where the marks go, and where the axis marks it. Freeing one has to move both, or the numbers stop describing the picture. Look again at the freed population plot above. Its gridlines sit at different heights in every panel, and each panel’s labels sit beside its own. Shared, those same lines are drawn at one set of heights across all five. That is the whole of what a freed guide means: the lines a reader measures against belong to the scale that was freed.
The third is that a free scale is a panel’s, never a frame’s. play (Animation) splits rows the same way a facet does, so asking for this over frames is a sentence you can write, and it is refused:
render_svg(data(gm_all) + point + x(gdp) + y(life, free = TRUE) + play(year))Error:
! gog: `y(life, free = TRUE)` fits one scale per *panel*, and this plot has frames rather than panels. A frame replaces the one before it, so an axis refitted per frame would move under the data and the motion would be the scale's rather than the data's. Facet the plot to free a scale across panels, or leave the sequence on one scale.
gog: nothing was rendered. Fix the above, or set GOG_STRICT=0 to draw anyway.The reason is in the message. Panels sit beside each other and a reader compares them; a frame replaces the one before it, so an axis refitted every frame would move under the data and the motion on screen would be the scale’s rather than the data’s. The same freedom is a fair trade in space and a lie in time.
Free scales are refused in two more places, both for the same kind of reason. On a channel that is not a position there is no axis to free, only a legend, and one key decodes the whole plot. And with no facet there are no panels:
render_svg(data(gapminder_2007) + point + x(gdp) + y(life, free = TRUE))Error:
! gog: `y(life, free = TRUE)` fits one scale per panel, and this plot has one panel. Facet it — `plot | facet(<column>)` — or drop `free`.
gog: nothing was rendered. Fix the above, or set GOG_STRICT=0 to draw anyway.A layer whose table does not have the facet column is drawn in full in every panel. That is how a shared reference belongs behind small multiples:
world_median <- data.frame(
gdp = c(min(gapminder_2007$gdp), max(gapminder_2007$gdp)),
life = rep(median(gapminder_2007$life), 2)
)
data(gapminder_2007) + point + x(gdp) + y(life) +
data(world_median) + line + style(color = "gray", size = 1) |
facet(continent)The gray line, the world’s median life expectancy, repeats in every panel, so each continent is read against the same anchor. The engine reports this as an assumption rather than doing it in silence: the table world_median has no continent column, so its layer is drawn everywhere.
A facet variable names the panels, so it must be a category column: a number is a position along an axis, not a name for a frame. Faceting by a numeric column refuses with direction:
render_svg(data(gapminder_2007) + point + x(gdp) + y(life) | facet(population))Error:
! gog: `facet(population)` splits on a number column, but a facet variable names the panels, so it must be a category column. Make `population` text — in R, `factor(population)` — or cut it into named groups first.
gog: nothing was rendered. Fix the above, or set GOG_STRICT=0 to draw anyway.The same judgment shape and group make about a mark’s features, applied to the plot’s outer frame. In R, factor(), or any explicit cut into named groups, is the way to say which panels you mean.
Trellis named that cut a shingle, and lattice still builds one with shingle() or equal.count(). A shingle’s intervals are allowed to overlap, so one row can appear in two panels, which smooths the run of panels at the price of counting some rows twice. Cutting the column yourself with cut() gives up the smoothing and puts the boundaries somewhere a reader can check them, in the strip labels.
Faceting joins with the operators, never with +; a facet is a property of the whole plot’s frame, not one more encoding in the chain. Writing + facet(continent) refuses and points at | and /.