Plotting lines connecting points

Question

I know there is another very similar question, but I could not extract the information I need from it.

plotting lines in pairs

I have 4 points in the (x,y) plane: x=[x1,x2,x3,x4] and y=[y1,y2,y3,y4]

x=[-1 ,0.5 ,1,-0.5]
y=[ 0.5,  1, -0.5, -1]

Now, I can plot the four points by doing:

import matplotlib.pyplot as plt

plt.plot(x,y, 'ro')
plt.axis('equal')
plt.show()

But, apart from the four points, I would like to have 2 lines:

1) one connecting (x1,y1) with (x2,y2) and 2) the second one connecting (x3,y3) with (x4,y4).

This is a simple toy example. In the real case I have 2N points in the plane.

How can I get the desired output: for points with two connecting lines ?

Thank you.

Answer 1

You can just pass a list of the two points you want to connect to plt.plot. To make this easily expandable to as many points as you want, you could define a function like so.

import matplotlib.pyplot as plt

x=[-1 ,0.5 ,1,-0.5]
y=[ 0.5,  1, -0.5, -1]

plt.plot(x,y, 'ro')

def connectpoints(x,y,p1,p2):
    x1, x2 = x[p1], x[p2]
    y1, y2 = y[p1], y[p2]
    plt.plot([x1,x2],[y1,y2],'k-')

connectpoints(x,y,0,1)
connectpoints(x,y,2,3)

plt.axis('equal')
plt.show()

Note, that function is a general function that can connect any two points in your list together.

To expand this to 2N points, assuming you always connect point i to point i+1, we can just put it in a for loop:

import numpy as np
for i in np.arange(0,len(x),2):
    connectpoints(x,y,i,i+1)

In that case of always connecting point i to point i+1, you could simply do:

for i in np.arange(0,len(x),2):
    plt.plot(x[i:i+2],y[i:i+2],'k-')

Answer 2

I realize this question was asked and answered a long time ago, but the answers don't give what I feel is the simplest solution. It's almost always a good idea to avoid loops whenever possible, and matplotlib's plot is capable of plotting multiple lines with one command. If x and y are arrays, then plot draws one line for every column.

In your case, you can do the following:

x=np.array([-1 ,0.5 ,1,-0.5])
xx = np.vstack([x[[0,2]],x[[1,3]]])
y=np.array([ 0.5,  1, -0.5, -1])
yy = np.vstack([y[[0,2]],y[[1,3]]])
plt.plot(xx,yy, '-o')

Have a long list of x's and y's, and want to connect adjacent pairs?

xx = np.vstack([x[0::2],x[1::2]])
yy = np.vstack([y[0::2],y[1::2]])

Want a specified (different) color for the dots and the lines?

plt.plot(xx,yy, '-ok', mfc='C1', mec='C1')

Answer 3

Use the matplotlib.arrow() function and set the parameters head_length and head_width to zero to don't get an "arrow-end". The connections between the different points can be simply calculated using vector addition with: A = [1,2], B=[3,4] --> Connection between A and B is B-A = [2,2]. Drawing this vector starting at the tip of A ends at the tip of B.

import numpy as np
import matplotlib.pyplot as plt
from matplotlib import style
style.use('fivethirtyeight')


A = np.array([[10,8],[1,2],[7,5],[3,5],[7,6],[8,7],[9,9],[4,5],[6,5],[6,8]])


fig = plt.figure(figsize=(10,10))
ax0 = fig.add_subplot(212)

ax0.scatter(A[:,0],A[:,1])


ax0.arrow(A[0][0],A[0][1],A[1][0]-A[0][0],A[1][1]-A[0][1],width=0.02,color='red',head_length=0.0,head_width=0.0)
ax0.arrow(A[2][0],A[2][1],A[9][0]-A[2][0],A[9][1]-A[2][1],width=0.02,color='red',head_length=0.0,head_width=0.0)
ax0.arrow(A[4][0],A[4][1],A[6][0]-A[4][0],A[6][1]-A[4][1],width=0.02,color='red',head_length=0.0,head_width=0.0)


plt.show()

Answer 4

With the code below you can create multiple lines by connecting points thanks to their coordinates :

import matplotlib.pyplot as plt

# 1st line
point_1 = [1,3]
point_2 = [2,6]

# 2nd line
point_3 = [4,6]
point_4 = [1,2]

x_values = [[point_1[0], point_3[0]],[point_2[0], point_4[0]]]
y_values = [[point_1[1], point_3[1]],[point_2[1], point_4[1]]]

plt.plot(x_values, y_values, 'red')
plt.show()

---

Result

Answer 5

I would prefer LineCollection in matplotlib. See following minimum code:

import matplotlib.pyplot as plt
from matplotlib.collections import LineCollection

x = np.arange(100)
# Here are many sets of y to plot vs. x
ys = x[:50, np.newaxis] + x[np.newaxis, :]

segs = np.zeros((50, 100, 2))
segs[:, :, 1] = ys
segs[:, :, 0] = x

# We need to set the plot limits.
fig, ax = plt.subplots(1, 1, figsize=(7.2, 7.2))
ax.set_xlim(x.min(), x.max())
ax.set_ylim(ys.min(), ys.max())

line_segments = LineCollection(segs)
ax.add_collection(line_segments)

A more detailed example can be found here.