How to Plot Horizontal and Vertical Line in Matplotlib

axhline
andaxvline
to Plot Horizontal and Vertical Lines in Matplotlib 
hlines
andvlines
to Plot Horizontal and Vertical Line in Matplotlib  Conclusion of Drawing Horizontal and Vertical Lines in Matplotlib
In this tutorial, we will introduce two methods to draw horizontal and vertical lines in Matplotlib. These two methods use different coordinates in Matplotlib.
axhline()
/axvline()
hlines()
/vlines()
axhline
and axvline
to Plot Horizontal and Vertical Lines in Matplotlib
axhline
to Plot a Horizontal Line
matplotlib.pyplot.axhline(y=0, xmin=0, xmax=1, hold=None, **kwargs)
axhline
plots a horizontal line at the position of y
in data coordinate of the horizontal line, starting from xmin
to xmax
that should be between 0.0
and 1.0
, where 0.0
is the far left of the plot and 1.0
is the far right of the plot.
from matplotlib import pyplot as plt
xdata = list(range(10))
ydata = [_*2 for _ in xdata]
plt.plot(xdata, ydata, 'b')
plt.axhline(y=5, xmin=0.1, xmax=0.9)
plt.grid()
plt.show()
axvline
to Plot a Vertical Line
Similarly,
matplotlib.pyplot.axvline(x=0, ymin=0, ymax=1, hold=None, **kwargs)
axvline
plots a vertical line at the position of x
in data coordinate of the vertical line, starting from ymin
to ymax
that should be between 0.0
and 1.0
, where 0.0
is the bottom of the plot and 1.0
is the top of the plot.
from matplotlib import pyplot as plt
xdata = list(range(10))
ydata = [_*2 for _ in xdata]
plt.plot(xdata, ydata, 'b')
plt.axvline(x=5, ymin=0.1, ymax=0.9)
plt.grid()
plt.show()
As stated above, xmin
/ xmax
and ymin
/ ymax
are referred to the plot, but not to the data lines.
Therefore, if we zoom in or out the plot, the starting and ending points of the horizontal and vertical lines will be updated with the reference to the data coordinate, but stick to the relative positions in the plot coordinate. We could check the below animation to have better a understanding.
hlines
and vlines
to Plot Horizontal and Vertical Line in Matplotlib
If we want the plotted horizontal and vertical lines will change automatically to keep the relative position to the data coordinate, we need to use hlines
and vlines
to plot lines.
hlines(y, xmin, xmax)
Here, y
, xmin
and xmax
are the values of the data coordinate.
vlines(x, ymin, ymax)
Here, x
, ymin
and ymax
are the values of the data coordinate.
Let’s see the example codes below.
from matplotlib import pyplot as plt
xdata = list(range(10))
ydata = [_*2 for _ in xdata]
plt.plot(xdata, ydata, 'b')
plt.hlines(y=5, xmin=0, xmax=10)
plt.vlines(x=5, ymin=0, ymax=20)
plt.grid()
plt.show()
Conclusion of Drawing Horizontal and Vertical Lines in Matplotlib
If you need the line to be referred to the plot, axhline
and axvline
should be the better option.
If you prefer the line to stick to the data coordinate, hlines
and vlines
are the better choices.