AB line

from plotnine import ggplot, aes, geom_point, labs, geom_abline, facet_grid
from plotnine.data import mpg

geom_abline() draws a line when the yintercept and gradient is supplied, and is useful as a guide.

# inspect the data
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

It’s useful to use geom_abline() with some data, so we start with a basic scatter plot:

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

Now layer a line over the scatter plot using geom_abline(). geom_abline() requires inputs for the slope (default slope is 1) and y-intercept (default value is [0,0]).

(
    ggplot(mpg, aes(x="displ", y="hwy"))
    + geom_point()
    + geom_abline(
        intercept=45,  # set the y-intercept value
        slope=-5,  # set the slope value
    )
    + labs(x="displacement", y="horsepower")
)

You can plot two lines on one plot:

(
    ggplot(mpg, aes(x="displ", y="hwy"))
    + geom_point()
    + geom_abline(
        intercept=[45, 45],  # add many lines to a plot using a list for the y-intercepts...
        slope=[-5, -7],  # ... and for the slopes
    )
    + labs(x="displacement", y="horsepower")
)

You can change the look of the line:

(
    ggplot(mpg, aes(x="displ", y="hwy"))
    + geom_point()
    + geom_abline(
        intercept=45,
        slope=-5,
        color="blue",  # set line colour
        size=2,  # set line thickness
        linetype="dashed",  # set line type
    )
    + labs(x="displacement", y="horsepower")
)

geom_abline() can be used with a facet plot:

(
    ggplot(mpg, aes(x="displ", y="hwy"))
    + geom_point()
    + geom_abline(intercept=45, slope=-5)  # add a line ...
    + facet_grid("drv")  # ... to a facet plot.
    + labs(x="displacement", y="horsepower")
)