Strange plot created based on two numpy arrays and matplotlib

Question

Now I just want to plot a line graph based on two numpy arrays. My x and y are both two (150,1) arrays. After running the following code:

plt.plot(x,y)

What I get is: Line graph based on two numpy arrays

Hence I am so confused. What do those connected lines represent? I just want one line which goes through all of the points. Any help would be appreciated!

For the dataset, X is just a fixed (150,1) numpy array and y is computed based on the following polynomial function:

def PolyCoefficients(x, coeffs):
""" Returns a polynomial for ``x`` values for the ``coeffs`` provided.

The coefficients must be in ascending order (``x**0`` to ``x**o``).
"""
o = len(coeffs)
y = []
for i in range(len(x)):
    value = 0
    for j in range(o):
        value += coeffs[j]*x[i]**j
    y.append(value)
return y

The coefficients have been computed and what I want is just a line go through each point of (x,y)

Joe · Accepted Answer · 2018-11-23 07:03:00Z

1

The pairs of x and y represent the points on your graph. With plt.plot() you join the points with a line. If the array x is not in order, what you have is a line that goes back and forward across the graph. To avoid this you should order the x array, and the y accordly. Try with:

new_x, new_y = zip(*sorted(zip(x, y)))
plt.plot(new_x,new_y)

edited Nov 23, 2018 at 7:03

answered Nov 23, 2018 at 6:53

Joe

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3 Comments

Bright Chang Over a year ago

Thank you for your answer. But actually my x is fixed and y is just the output from a polynomial function based on x. The coefficients have already been computed... Does the order of these two arrays change during the computation?

Joe Over a year ago

If you have just these 2 lists, order them for the graph. I've added some code to the answer, try it and see if it works

Joe Over a year ago

You are welcome, dont forget to acecpt the answer :) meta.stackexchange.com/questions/5234/…

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