Python doesn't plot the graph

January 31, 2024

i have a little problem with matplotlib and python. So my problem is the line don’t appear in the plot. I am trying to make a graph of a custom function. My code is here bellow:

import scipy
from numpy import arange
import matplotlib.pyplot as plt


def integral_function(x, a, b, c): # defines the function
    return a*x**2 + b*x + c
for r in arange(0,5,0.01): # defines the range and step
    x = r
y =scipy.integrate.quad(integral_function, 0, x, args=(2,3,4))[0] # computes the integral for the first list of constants
y1 = scipy.integrate.quad(integral_function, 0, x, args=(2,1,1))[0] # computes the integral for the second list of constants
plt.plot(r, y) # plots the results of the first list of constants
plt.plot(r, y1) # plots the results of the  second list of constants
plt.xlabel("r values")
plt.ylabel("values of integration")
plt.legend(["for a,b,c = 2,3,4","for a,b,c = 2,1,1"])
plt.title("integration of ax^2 + bx + c")
plt.show()

Everytime that i run the code the graph appears without the lines. Please could someone can help me ? Thank you for the attention 🙂

>Solution :

This is because you are trying to plot a point using plot() function. You can either use scatter() function to draw the point or pass a color and dot sign to plot() function.

import scipy
from numpy import arange
import matplotlib.pyplot as plt


def integral_function(x, a, b, c): # defines the function
    return a*x**2 + b*x + c

for r in arange(0,5,0.01): # defines the range and step
    x = r
    y =scipy.integrate.quad(integral_function, 0, x, args=(2,3,4))[0] # computes the integral for the first list of constants
    y1 = scipy.integrate.quad(integral_function, 0, x, args=(2,1,1))[0] # computes the integral for the second list of constants
    plt.plot(r, y,'b.') # plots the results of the first list of constants
    plt.plot(r, y1,'r.') # plots the results of the  second list of constants
plt.xlabel("r values")
plt.ylabel("values of integration")
plt.legend(["for a,b,c = 2,3,4","for a,b,c = 2,1,1"])
plt.title("integration of ax^2 + bx + c")
plt.show()

The output will be :

Is there a better way to draw this plot? Absolutely! It’s recommended to calculate all values before plotting, then plotting all values at once.

import scipy
from numpy import arange
import matplotlib.pyplot as plt


def integral_function(x, a, b, c): # defines the function
    return a*x**2 + b*x + c
y = []
y1 = []
for r in arange(0,5,0.01): # defines the range and step
    x = r
    y.append(scipy.integrate.quad(integral_function, 0, x, args=(2,3,4))[0]) # computes the integral for the first list of constants
    y1.append(scipy.integrate.quad(integral_function, 0, x, args=(2,1,1))[0]) # computes the integral for the second list of constants

    
r = arange(0,5,0.01)
plt.plot(r, y,'b')
plt.plot(r, y1,'r')
plt.xlabel("r values")
plt.ylabel("values of integration")
plt.legend(["for a,b,c = 2,3,4","for a,b,c = 2,1,1"])
plt.title("integration of ax^2 + bx + c")
plt.show()

And here is the output of recommended method :