[Tutor] Plotting subsets of data

Colin Ross colin.ross.dal at gmail.com
Mon Dec 8 16:48:41 CET 2014

```Good afternoon,

I am using the following to code to plot the output from an optical encoder:

import numpy as np
from numpy import ma, logical_or
import pylab
from pylab import *
import matplotlib.pyplot as plt
from matplotlib.ticker import AutoMinorLocator
import sys

# Defining x and y minorlocator

xminorLocator=AutoMinorLocator()
yminorLocator=AutoMinorLocator()

# Load data from .txt file

data = np.loadtxt('2014_12_04-16_30_03.txt',skiprows = 0 ,usecols = (0,1))

print "\n Chopper Test for X-SPEC Prototype"
print "\n  Time (sec)     Pos (deg)"
print data

# Place column data in array to be plotted

time = np.array(data[:,0])
print "Time (sec):"
#print time

pos = np.array(data[:,1])
print "Position (deg):"
print pos

# Setting minor ticks

ax = plt.subplot(111)
ax.xaxis.set_minor_locator(xminorLocator)
ax.yaxis.set_minor_locator(yminorLocator)

#Plotting commands.

subplot(2,1,1)
plot(time, pos, 'ro')
title('Encoder Output')
ylabel('Pos (deg)')

subplot(2,1,2)
plot(t_small, pos, 'ro')
xlabel('Time (sec)')
ylabel('Pos (deg)')

show()

The desired result is a square wave, but this is not readily available from
the plot (see attached). For the subplot(2,1,2) I would like to plot the
output over a 5 second interval so that the behaviour becomes more evident.
Can someone please advise me as to the easiest way isolate the first 5
seconds of data?

Thank you.
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