在Python中绘制切线图（matplotlib）

Question

Today I decided to write simple program in Python, just to practice before exam. Firstly, I wanted to draw sin and cos plot, which wasn't so hard. But then, I decided to challenge myself and draw tangent plot.

import pylab as p

x= p.arange(-1.0,1.0,0.1)
y= (p.sin(2*p.pi*x)) / (p.cos(2*p.pi*x))

p.plot(x,y,'g-',lw=1)
p.show()

It returns... ugh... this:

As you can see, it looks more like ECK plot than tangent plot. Does anyone knows what's wrong?

Answer 1

If you increase the number of points in x ,

import pylab as p
import numpy as np
x = p.linspace(-1.0, 1.0, 1000)
y = (p.sin(2 * p.pi * x)) / (p.cos(2 * p.pi * x))
p.plot(x, y, 'g-', lw=1)
p.show()

you get something like this:

Notice how large the y-range is getting. Matplotlib is not able to show you much of the small values in the tangent curve because the range is so large.

The plot can be improved by ignoring the extremely large values near the asymptotes. Using Paul's workaround to handle asymptotes,

import pylab as p
import numpy as np
x = p.linspace(-1.0, 1.0, 1000)
y = (p.sin(2 * p.pi * x)) / (p.cos(2 * p.pi * x))

tol = 10
y[y > tol] = np.nan
y[y < -tol] = np.nan

p.plot(x, y, 'g-', lw=1)
p.show()

you get

Answer 2

import pylab as p

x= p.arange(-1.0,1.0,0.01)  # <- 0.01 step size.
y= (p.sin(2*p.pi*x)) / (p.cos(2*p.pi*x))
# y = p.tan(2*p.pi*x)
p.plot(x,y,'g-',lw=1)
p.ylim([-4, 4]) # <- Restricting the ylim so we don't see the ~inf values. 
p.show()

This will be the result if you don't set ylim. (the values approach infinity)

Result with setting ylim.