Matplotlib Examples: Drawing Straight Lines

Matplotlib Examples: Drawing Straight Lines

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Table of Contents

All code available online on this jupyter notebook

Horizontal line

Using plt.axhline(height) is the easiest way to draw a horizontal line at a given height

import matplotlib.pyplot as plt

height = 5


horizontal-line-using-axhline Using axhline is the simplest way to draw a horizontal line in matplotlib

Vertical line

Use plt.axvline() to draw a vertical line at a given position:

import matplotlib.pyplot as plt

x_level = 5


vertical-line-using-axvline Use axvline to draw a vertical line anywhere

Vertical line, string axis

If your x-axis has strings or categories instead of numbers you can also call plt.axvline()

import matplotlib.pyplot as plt

# sample data
days   = ['sun','mon','tue','wed','thu','fri','sat']
values = [1    , 4   , 4   , 8   , 3   , 4   , 3   ]

# plotting bars for comparison, values)

plt.axvline('thu', color='red')

image-before-no-lines BEFORE: a plot with string
labels in the x-axis
after-line-drawn-on-string-axis AFTER: call plt.axvline('thu') to
draw a vertical line on 'Thu'

45 degree line passing through point

Use plt.axline(point, slope)

The slope is given in the units of the chart so you need to calcualte it based on your points:

import matplotlib.pyplot as plt
import numpy as np

# note that the x-axis is 10 times as large (in magnitude)
# as the y-axis
xs = np.random.uniform(0,100,100)
ys = np.random.uniform(0,10,100)

# just to populate the plot

# An angle of 45 degrees means a slope of 1 (relatively)
slope = (ys.max() - ys.min()) / (xs.max() - xs.min())


line-at-given-angle-passing-through-through-given-point We first need to calculate the slope becase it depends on
the magnitude of the axes.
Then we just call plt.axline() passing the point and the slope.

Using Coordinates

Use plt.axline((x1,y1),(x1,y2)) to draw an infinitely long line passing through (x1,y1) and (x2,y2)

Example: Draw a line passing through (1,2) and (5,7)

import matplotlib.pyplot as plt


line-plotted-with-axline Use can also use plt.axline() to draw
arbitrary lines from one point to another

Dotted line

You can pass any of the styles here to "linestyle"

Just pass linestyle='dotted' to either plt.axvline(), plt.axhline() or plt.axline():

import matplotlib.pyplot as plt


vertical-line-dotted Horizontal dotted line

Dashed line

Again, everything here can be used in "linestyle"

Pass linestyle='dashed' to any of plt.axhline(), plt.axvline() or plt.axline():

import matplotlib.pyplot as plt


vertical-line-dashed Dashed line, vertical


Dialogue & Discussion