Matplotlib: plotting discrete values

Question

I am trying to plot the following !

from numpy import *
from pylab import *
import random

for x in range(1,500):
    y = random.randint(1,25000)
    print(x,y)   
    plot(x,y)

show()

However, I keep getting a blank graph (?). Just to make sure that the program logic is correct I added the code print(x,y), just the confirm that (x,y) pairs are being generated.

(x,y) pairs are being generated, but there is no plot, I keep getting a blank graph.

Any help ?

Arkapravo, i have no problem w/ the accepted answer; i just wanted to mention that this five-line template: from matplotlib import pyplot as PLT; fig=PLT.figure(); ax1=fig.add_subplot(111); ax1.plot(x, y); PLT.show(), is a quick way to get a working x-y plot 98% of the time. ('x', 'y' are lists or 1D Numpy arrays, btw). — doug
– doug, Commented Apr 7, 2010 at 8:19
@doug: you can go the same thing in a simpler way: from matplotlib import pyplot at plt; plt.plot(x, y); plt.show(). — Eric O. Lebigot
– Eric O. Lebigot, Commented Apr 7, 2010 at 9:20
@doug: Thanks ! ... I accepted Daniel G's answer since it immediately solved all my issues ! .... :) .. too bad 2 answers cannot be chosen ! — Arkapravo
– Arkapravo, Commented Apr 7, 2010 at 10:30

Daniel G · Accepted Answer · 2010-04-07 07:55:07Z

5

First of all, I have sometimes had better success by doing

from matplotlib import pyplot

instead of using pylab, although this shouldn't make a difference in this case.

I think your actual issue might be that points are being plotted but aren't visible. It may work better to plot all points at once by using a list:

xPoints = []
yPoints = []
for x in range(1,500):
    y = random.randint(1,25000)
    xPoints.append(x)
    yPoints.append(y)
pyplot.plot(xPoints, yPoints)
pyplot.show()

To make this even neater, you can use generator expressions:

xPoints = range(1,500)
yPoints = [random.randint(1,25000) for _ in range(1,500)]
pyplot.plot(xPoints, yPoints)
pyplot.show()

answered Apr 7, 2010 at 7:55

Daniel G

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3 Comments

doug Over a year ago

+1 for 'import pyplot'--obviously for those former Matlab users, pylab is a massive convenience, but the almost interchangeable use of these two dialects in the docs, examples, etc. made Matplotlib, brilliant as it is, much harder for me to learn.

Arkapravo Over a year ago

Many thanks dudes ! I guess I had last dabbled with matplotlib, sometimes in October 2009 , certain needs came up that I had to come back to it in April 2010 .

spacecowboy Over a year ago

yPoints = list(map(lambda: random.randint(1,25000), xPoints)) and no need to keep track of an extra variable.

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