Need help setting y range for matplotlib for scientific data

Question

I have a simple text file with scientific data, with one data item on each line. I want to plot it simply as sequential items as y-axis. I call this: list_y

Within the Python script I create a list that is a simple sequential number list for the x-axis. I call this: list_x

When I plot, using the matplotlib calls, the same as dozens of examples that I have read, I get unexpected results. Matplotlab does not seem to give a y-axis from the smallest-to-largest of my y value data items. It seems to plot number pairs - not what I would call a "graph".

I have read various examples and watched Youtube demonstrations with the exact code that I am using, and those examples all give a result that I would expect.

# -*- coding: utf-8 -*-
"""
Created on Tue Mar 10 12:18:57 2020

@author: usera
"""

from matplotlib import pyplot as plt

# data in data01a.txt is:  0.1  0.22  0.33  0.5  0.6  0.66  1  .9  

data_han = open('data01a.txt', 'r')
list_y = data_han.readlines()
data_han.close()

y_size = len(list_y)

ii = 0
list_x = []

#follow while stmt creates a numeric list for x-axis
while ii < y_size:
    list_x.append(ii)
    ii += 1

plt.xlabel('Sequence')
plt.ylabel('Value')

plt.legend()

plt.grid('True')

#following 3 lines of code commented out 
#since it causes strange results
#axes = plt.gca()
#axes.set_xlim(0,10)
#axes.set_ylim(0,1)

#following commented out code does not give correct x axis
#and no line is plotted at all
#plt.axis(0,10,0,1)

#following commented out code does not give correct y axis
#plt.xlim(0,10)
#plt.ylim(0,1)

plt.plot(list_x, list_y, label='Test Plot')

plt.savefig('test01a.png')

plt.show()

Any help is much appreciated.

Thank you.

Answer 1

A very "quick and dirty" approach would be something like the following:

#!/usr/bin/env python3
# -*- coding: utf-8 -*-

from matplotlib import pyplot as plt

y_data = [0.1, 0.22, 0.33, 0.5, 0.6, 0.66, 1, 0.9]
y_data = sorted(y_data)

x_data = [i+1 for i in range(len(y_data))]

plt.plot(x_data, y_data)
plt.show()

I stripped off, every part not needed for a very basic example. So, feel free to add any configuration for grid, title or labels later.

I assumed, you are able to read and parse the data from your input file into a list of floats called y_data. In addition, I assumed that you want to sort the y-values from lowest to highest, as you have stated:

does not seem to give a y-axis from the smallest-to-largest of my y value data items

If you want to keep the original order of your y_data, simply remove the line y_data = sorted(y_data).

Next thing we should elaborate on, is plotting with plt.plot(). According to the docs, providing x-values is optional and we are allowed to provide y-values without any x-values. When doing so, the element index of the corresponding y-value will be used as the x-value. This would result in a value range of 0...N-1. You might want to have 1...N which we can achieve by calculating a very basic list of x-values based on the length list containing the y-values y_data. By doing so, we can pass both lists x_data and y_data to plt.plot() and get the desired output:

The output without explicitly given x-values would be quite similar, but 0-based instead of 1-based on the x-axis. Change plt.plot(x_data, y_data) into plt.plot(y_data) to see the difference: