Facet grid


from plotnine import ggplot, aes, geom_point, labs, facet_grid, theme, element_text, element_rect
from plotnine.data import mpg

facet_grid() is used to form a grid of plots, where the rows and columns of the grid are set by the faceting variables. It is useful for visualising two discrete variables.

mpg.head()

	manufacturer	model	displ	year	cyl	trans	drv	cty	hwy	fl	class
0	audi	a4	1.8	1999	4	auto(l5)	f	18	29	p	compact
1	audi	a4	1.8	1999	4	manual(m5)	f	21	29	p	compact
2	audi	a4	2.0	2008	4	manual(m6)	f	20	31	p	compact
3	audi	a4	2.0	2008	4	auto(av)	f	21	30	p	compact
4	audi	a4	2.8	1999	6	auto(l5)	f	16	26	p	compact

Basic scatter plot:

(
    ggplot(mpg, aes(x="displ", y="hwy"))
    + geom_point()
    + labs(x="displacement", y="horsepower")
)

Facet a discrete variable into rows:

(
    ggplot(mpg, aes(x="displ", y="hwy"))
    + geom_point()
    + facet_grid("drv")
    + labs(x="displacement", y="horsepower")
)

Facet a discrete variable into columns:

(
    ggplot(mpg, aes(x="displ", y="hwy"))
    + geom_point()
    + facet_grid(cols="cyl")
    + labs(x="displacement", y="horsepower")
)

Facet two discrete variables into rows and columns:

(
    ggplot(mpg, aes(x="displ", y="hwy"))
    + geom_point()
    + facet_grid("drv", "cyl")
    + labs(x="displacement", y="horsepower")
)

To change the plot order of the rows or columns in the facet grid, use a categorical and reorder the levels of the faceting variable in the data.

# Convert to categorical and re-order categories
mpg["drv"] = mpg["drv"].astype("category").cat.reorder_categories(["f", "r", "4"])

# facet plot with reorded drv category
(
    ggplot(mpg, aes(x="displ", y="hwy"))
    + geom_point()
    + facet_grid("drv", "cyl")
    + labs(x="displacement", y="horsepower")
)

You can choose if the scale of x- and y-axes are fixed or variable by using the scales argument within the facet_grid() command:

(
    ggplot(mpg, aes(x="displ", y="hwy"))
    + geom_point()
    + facet_grid("drv", scales="free")
    + labs(x="displacement", y="horsepower")
)

You can add additional information to your facet labels, by using the labeller argument within the facet_grid() command. Below we use labeller = 'label_both' to include the column name in the facet label.

(
    ggplot(mpg, aes(x="displ", y="hwy"))
    + geom_point()
    + facet_grid("drv", labeller="label_both")
    + labs(x="displacement", y="horsepower")
)

You can add two discrete variables to a facet:

# add additional column for plotting exercise
mpg["transmission"] = mpg["trans"].map(
    lambda x: "auto" if "auto" in x else "man" if "man" in x else ""
)

# inspect new column transmission which identifies cars as having an automatic or manual transmission
mpg.head()

	manufacturer	model	displ	year	cyl	trans	drv	cty	hwy	fl	class	transmission
0	audi	a4	1.8	1999	4	auto(l5)	f	18	29	p	compact	auto
1	audi	a4	1.8	1999	4	manual(m5)	f	21	29	p	compact	man
2	audi	a4	2.0	2008	4	manual(m6)	f	20	31	p	compact	man
3	audi	a4	2.0	2008	4	auto(av)	f	21	30	p	compact	auto
4	audi	a4	2.8	1999	6	auto(l5)	f	16	26	p	compact	auto

# facet plot with two variables on one facet
(
    ggplot(mpg, aes(x="displ", y="hwy"))
    + geom_point()
    + facet_grid(["drv", "transmission"])  # use a list to add additional faceting variables
    + labs(x="displacement", y="horsepower")
)

Facet labels can be rotated to make them easier to read using strip_text_y = element_text(angle = 0) for row labels within the theme() command (use strip_text_x = element_text(angle = 0) for column labels).

If the labels do not fit in the strip, adjust the width of the strip using strip_background_y for rows (use strip_background_x for columns). You may also need to adjust the text position so it fits in the strip: adjust the horizontal text position in the strip by specifying ha in element_text()(specify va in element_text() to adjust the vertical text position).

You can also change the colour of the strip by specifying color in element_text() .

(
    ggplot(mpg, aes(x="drv", y="model"))
    + geom_point()
    + facet_grid("manufacturer", scales="free")
    + theme(
        strip_text_y=element_text(angle=0),   # change facet text angle
        strip_background_y=element_rect(fill="#969dff"),   # change background colour of facet background
        figure_size=(6, 15),  # adjust width & height of figure to fit y-axis
    )
    + labs(x="displacement", y="")
)