CODEWITHSUNDEEP
pythonmatplotlib
user7722318
user7722318

Reputation:

Draw arrow in matplotlib

Consider the following example: This code will create an circle with center 180, 150 and radius 15. Further a point will be added at position a,b. 

%matplotlib inline
import matplotlib.pyplot as plt
import math
import numpy as np

def get_circle(x, y, r):
    """ Get a cirlce with a specific position and radius"""
    result = plt.Circle((x, y), r, color='k', fill=False)
    return result

def rpoint(x, y, r, deg):
    """ Get a point on a radius at deg degrees."""
    rang = np.deg2rad(deg)
    x1 = x + r * np.sin(rang)
    y1 = y + r * np.cos(rang)
    return (x1, y1)

fig, ax = plt.subplots(1, 1, figsize=(6,6))

ax.set_aspect('equal', adjustable='datalim')
ax.axes.get_xaxis().set_visible(False)
ax.axes.get_yaxis().set_visible(False)

x1, y1, r = 180, 150, 15
ax.add_patch(get_circle(x1, y1, r))
a, b = rpoint(x1, y1, r, 180)
plt.plot(a,b,'ok')

enter image description here

Now i want simply to add an arrow that shows in the direction of the black dot with half the radius. From the docs i tried to add the desired arrow in the following way.

plt.arrow(x1, y1, a, b)

or

plt.arrow(x1, y1, x1 + abs(x1 - a), y1 + abs(y1 - b))

Both ways result in the same result:

enter image description here

The arrows direction and length is completely wrong even though i used the correct coordinates a,b. What am I doing wrong?

Upvotes: 0

Views: 2420

Answers (2)

Liris
Liris

Reputation: 1511

The second and third arguments of plt.arrow are dx and dy and not the coordinates of the ending points (from the doc).

Therefore use plt.arrow(x1, y1, a - x1, (b - y1)/2). Note that the division by two comes from the fact that you want the arrow to end at half the radius.

Upvotes: 1

Josh Friedlander
Josh Friedlander

Reputation: 11657

I don't fully understand what's happened, but the origin of your circle is (0, 0), so you need to give it a negative value. This works:

plt.arrow(x1, y1, 0, -r / 2, head_width=1, head_length=1)

Or in terms of your variables:

plt.arrow(x1, y1, a - x1, (b - y1) / 2, head_width=1, head_length=1)

Upvotes: 2

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