Matplotlib regression scattered plot using Python?

Question

enter image description here

My Question

I tried regression using curve_fit function from scipy module, and now I am getting a scattered kind of plot, I can't figure it out, Why I am getting a scattered plot here ?

My Code

def scipyFunction(x,y):
    plot(x, y, 'o', label='Original data', markersize=10)

    x = [float(xn) for xn in x] #every element (xn) in x becomes a float
    y = [float(yn) for yn in y] #every element (yn) in y becomes a float
    x = np.array(x) #tranform data into numpy array
    y = np.array(y) #transform data into numpy array

    def functionForScipy(x,a,b,c,d):
        return a*x**3 + b*x**2 + c*x + d 

    #make the curve_fit
    popt,pcov = curve_fit(functionForScipy,x,y)

    '''
    The result is :
    popt[0] = a, popt[1] = b, popt[2] = d of the function,
    so f(x) = popt[0]*x**3 + popt[1]*x**2 + popt[2]*x + popt[3].
    '''

    print(popt)

    plt.plot(x, popt[0]*x**3 + popt[1]*x**2 + popt[2]*x + popt[3], label="Fitted Curve")  # same as lin eabove

    plt.legend(loc='upper left')
    plt.show()

x and y plot is like this :

enter image description here

Are you asking why your green line is not straight but is instead jumping around the place? — Ffisegydd
– Ffisegydd, Commented Aug 12, 2014 at 13:13

Ffisegydd · Accepted Answer · 2014-08-12 13:25:17Z

2

I suspect this occurs because the values in your x array are not monotonically increasing (that is to say that the each subsequent number is larger than the last).

You need to sort your x values before you plot them otherwise they will be all over the place, as shown in the example below.

import numpy as np
import matplotlib.pyplot as plt

def func(x):
    return x**2

x = np.array([0, 5, 2, 1, 3, 4])
y = func(x)

plt.plot(x, y, 'b-', label='Unsorted')

x.sort()
y = func(x)

plt.plot(x, y, 'r-', label='Sorted')

plt.legend()

plt.show()

Example

answered Aug 12, 2014 at 13:25

Ffisegydd

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Comments

j-i-l · Accepted Answer · 2014-08-12 13:37:49Z

1

Probably your x and y are not sorted.

Don't forget the apply the same sorting you do on x on y as well. To achieve this zip is very handy. You could add the following at the beginning of your function:

comb = zip(x,y)
comb.sort(key=lambda x:x[0])  #sort according to x
x, y = zip(*comb)  #both x and y are sorted now according to x

edited Aug 12, 2014 at 13:37

answered Aug 12, 2014 at 13:31

j-i-l

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3 Comments

Akash Rana Over a year ago

Thank you so much, just a minor mistake in here zip object cannot be sorted like that, it has to be converted into list, thus comb = list(zip(z,y)), and then apply sort..thanks anyway.. :)

j-i-l Over a year ago

@AkashRana run print type(zip([1,2,3],[2,3,1])) ;)

j-i-l Over a year ago

Just to clarify, from python.org: zip([iterable, ...]) This function returns a list of tuples.

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