CODEWITHSUNDEEP
pythonpython-3.xpandasmatplotlibjupyter-notebook
BeigeSponge
BeigeSponge

Reputation: 37

Matplotlib displaying axes but not datapoints

I'm trying to plot a velocity profile, with the following code. The axes are plotted, however no data points are plotted.

import pandas as pd
from matplotlib import pyplot as plt

n = 0.4
k = 53
d = 0.000264
r = 0.000132
p = 15000
u = (n/n+1)*(p*1/2*k)**(1/n)*(d**((n+1)/n) - r**((n+1)/n))
plt.plot(u)

Graph produced: Graph produced

Upvotes: 0

Views: 199

Answers (1)

JohanC
JohanC

Reputation: 80319

First off, note that (p*1/2*k) is a very confusing way to write a multiplication. In (about all) programming languages, the multiplications and divisions are done left to right, so, (p*1/2*k) equals (p*k/2) while perhaps you meant (p/(2*k)).

When plotting a 2D graph, you have to think what you want in the x direction, and what in the y direction. As you only give an x, there is nothing to plot. Also, plot default want to draw lines and for a line at least 2 xy pairs are needed. To only draw a point, plot accepts a third parameter, for example 'ro' to represent a red dot. Supposing u is meant to be the y direction and you don't have an x, you could give it a zero:

plt.plot(0, u, 'ro')

Now, probably you want to draw a curve of u for different values of some x. As in your equation there is no x nor a t, it is hard for me to know what you would like to see on the horizontal direction.

Let's suppose you want to show u as a function of d and that d goes from 0.0 to 0.0005. Typically, with numpy you create a sequence of values for d, lets say split into 200 small intervals: d = np.linspace(0.0, 0.0005, 200). Then, there is the magick of numpy, that when you write u = f(d), numpy makes an array for u with as many entries as d.

Example:

import numpy as np
from matplotlib import pyplot as plt

n = 0.4
k = 53
d = np.linspace(0.0, 0.0005, 200)  # 0.000264
r = 0.000132
p = 15000
u = (n / n + 1) * (p * 1 / 2 * k) ** (1 / n) * (d ** ((n + 1) / n) - r ** ((n + 1) / n))
plt.plot(d, u)
plt.show()

the plot

Upvotes: 1

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