Facet grid

In [1]:


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.

In [2]:

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:

In [3]:

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

Facet a discrete variable into rows:

In [4]:

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

Facet a discrete variable into columns:

In [5]:

(
    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:

In [6]:

(
    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, reorder the levels of the faceting variable in the data.

In [7]:

# re-order categories
mpg["drv"] = mpg["drv"].cat.reorder_categories(["f", "r", "4"])

In [8]:

# 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:

In [9]:

(
    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.

In [10]:

(
    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:

In [11]:

# 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 ""
)

In [12]:

# 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

In [13]:

# 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() .

In [14]:

(
    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="")
)