stackoverflow comment (https://stackoverflow.com/questions/25227100/best-way-to-plot-an-angle-between-two-lines-in-matplotlib/25228427#25228427)

vector_angle.py

import math

import matplotlib.pyplot as plt
from matplotlib.lines import Line2D

from matplotlib.patches import Arc


def get_angle_plot(line1, line2, offset=1, color=None, origin=(0, 0), len_x_axis = 1, len_y_axis = 1):

    l1xy = line1.get_xydata()

    # Angle between line1 and x-axis
    slope1 = (l1xy[1][1] - l1xy[0][1]) / float(l1xy[1][0] - l1xy[0][0])
    angle1 = abs(math.degrees(math.atan(slope1))) # Taking only the positive angle

    l2xy = line2.get_xydata()

    # Angle between line2 and x-axis
    slope2 = (l2xy[1][1] - l2xy[0][1]) / float(l2xy[1][0] - l2xy[0][0])
    angle2 = abs(math.degrees(math.atan(slope2)))

    theta1 = min(angle1, angle2)
    theta2 = max(angle1, angle2)

    angle = theta2 - theta1

    if color is None:
        color = line1.get_color() # Uses the color of line 1 if color parameter is not passed.

    return Arc(origin, len_x_axis*offset, len_y_axis*offset, 0, theta1, theta2, color=color, label = str(angle)+u"\u00b0")


def get_angle_text(angle_plot):
    angle = angle_plot.get_label()[:-1] # Excluding the degree symbol
    angle = "%0.2f"%float(angle)+u"\u00b0" # Display angle upto 2 decimal places

    # Get the vertices of the angle arc
    vertices = angle_plot.get_verts()

    # Get the midpoint of the arc extremes
    x_width = (vertices[0][0] + vertices[-1][0]) / 2.0
    y_width = (vertices[0][1] + vertices[-1][1]) / 2.0


    separation_radius = max(x_width/2.0, y_width/2.0)

    return [x_width + separation_radius, y_width + separation_radius, angle]


fig = plt.figure()

line_1 = Line2D([0,1], [0,4], linewidth=1, linestyle = "-", color="green")
line_2 = Line2D([0,4.5], [0,3], linewidth=1, linestyle = "-", color="red")

ax = fig.add_subplot(1,1,1)

ax.add_line(line_1)
ax.add_line(line_2)

angle_plot = get_angle_plot(line_1, line_2, 1)
angle_text = get_angle_text(angle_plot)
# Gets the arguments to be passed to ax.text as a list to display the angle value besides the arc

ax.add_patch(angle_plot) # To display the angle arc
ax.text(*angle_text) # To display the angle value

ax.set_xlim(0,7)
ax.set_ylim(0,5)

plt.legend()
plt.savefig('./vector_angle.png', dpi=300)

Excellent script. I might make one modification that allows you to deal with lines in different quadrants. Let me know if you think those would work for more situations. Thanks!

def get_angle_plot(line1, line2, offset=1, color=None, origin=(0, 0), 
                   len_x_axis = 1, len_y_axis = 1):
    
    l1xy = line1.get_xydata()
    
    # Angle between line1 and x-axis
    y1 = l1xy[1][1] - l1xy[0][1]
    x1 = l1xy[1][0] - l1xy[0][0]
    slope1 = y1 / float(x1)
    # Allows you to use this in different quadrants
    angle1 = math.degrees(math.atan2(y1, x1))
    
    l2xy = line2.get_xydata()
    
    # Angle between line2 and x-axis
    y2 = l2xy[1][1] - l2xy[0][1]
    x2 = l2xy[1][0] - l2xy[0][0]
    slope2 = y2 / float(x2)
    angle2 = math.degrees(math.atan2(y2, x2))
    
    theta1 = min(angle1, angle2)
    theta2 = max(angle1, angle2)
    
    angle = theta2 - theta1
    
    if color is None:
        color = line1.get_color() # Uses the color of line 1 if color parameter is not passed.
    
    return Arc(origin, len_x_axis*offset, len_y_axis*offset, 0, 
               theta1, theta2, color=color, 
               label = r'${:.4}^\circ$'.format(float(angle)))


def get_angle_text(angle_plot):
    angle = angle_plot.get_label()
    # angle = r'${:.4}^\circ$'.format(angle) # Display angle upto 2 decimal places
    
    # Get the vertices of the angle arc
    vertices = angle_plot.get_verts()
    
    # Get the midpoint of the arc extremes
    x_width = (vertices[0][0] + vertices[-1][0]) / 2.0
    y_width = (vertices[0][1] + vertices[-1][1]) / 2.0
    
    separation_radius = max(x_width/2.0, y_width/2.0)
    
    return [x_width + separation_radius, y_width + separation_radius, angle]

I'll let you know after reviewing your code.

I always get the following error for your code:

Traceback (most recent call last):
File "/home/lars/documents/ba/202311-ba-alsbach-rl_based_so_for_fibre_reinforced_concrete/05-Code/plotting/plotGenerationBA/test.py", line 72, in
angle_plot = get_angle_plot(line_1, line_2, 1)
File "/home/lars/documents/ba/202311-ba-alsbach-rl_based_so_for_fibre_reinforced_concrete/05-Code/plotting/plotGenerationBA/test.py", line 34, in get_angle_plot
return Arc(
TypeError: init() takes 4 positional arguments but 7 were given

Edit:

I fixed it:

exchange

Arc(origin, len_x_axis*offset, len_y_axis*offset, 0, 
               theta1, theta2, color=color, 
               label = r'${:.4}^\circ$'.format(float(angle)))

with

Arc(origin, len_x_axis*offset, len_y_axis*offset, angle = 0.0, 
               theta1, theta2, color=color, 
               label = r'${:.4}^\circ$'.format(float(angle)))

fixed it for me.