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I have tried to strip and get the data in the .txt file to allow me to plot a simple graph, but i can't seem to get the data into the format that i would like. Could someone guide me in the right direction?

Below is a short example of the data in the text file, and in python i am trying to .read() the text file, then plot a simple graph, using the headings in the text file itself if possible.

Date,Value
2016-03-31,0.7927
2016-03-30,0.7859    
2016-03-29,0.7843  
2016-03-24,0.7893    
2016-03-23,0.792    
2016-03-22,0.7897    
2016-03-21,0.7818    
2016-03-18,0.778    
2016-03-17,0.781    
2016-03-16,0.7855   
2016-03-15,0.7845

my python code that i have tried so far: (this won't be perfect code as i am still sorting through it!)

import numpy as np
import matplotlib.pyplot as plt


with open("EURGBP DATA.txt") as f:
data = f.read()

data = data.split('\n')

x = [row.split()[0] for row in data]
y = [row.split()[1] for row in data]

index = [i for i,val in enumerate(x)]

fig = plt.figure()
ax1 = fig.add_subplot(111)
ax1.set_title("Plot DAta")    
ax1.set_xlabel('x')
ax1.set_ylabel('y')
ax1.set_xticklabels(x)
ax1.plot(index ,y, c='r', label='the data')
leg = ax1.legend()
plt.locator_params(nbins=len(index)-1)
plt.show()
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3 Answers 3

Reset to default
4

With pandas (I use import pandas as pd below), this can actually be done with one line:

 pd.read_table('datafile.txt', parse_dates = True, index_col = 0, sep = ',').plot()

where the parse_dates keyword tells pandas to try to convert the index to datetime. The result looks like this: enter image description here

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1 Comment

Thanks, I tried this and it works perfectly! I wouldn't have the skillset as yet to write something so beautiful!
3

The DataFrame object in pandas already has a function plot() which is very helpful. Copying your example to the clipboard I could perform the plot just doing:

import pandas as pd
import matplotlib.pyplot as plt

df = pd.read_clipboard(delimiter=',')
df.plot()
ax = plt.gca()
ax.set_xticklabels(df.Date)
plt.savefig(filename='test.png')
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Comments

2

These lines split the data rows on spaces, not commas:

x = [row.split()[0] for row in data]
y = [row.split()[1] for row in data]

You need to specify the character to split on (whitespace characters are the default):

x = [row.split(',')[0] for row in data]
y = [row.split(',')[1] for row in data]

EDIT: additional data cleaning

If the data file has a trailing newline then 

y = [row.split(',')[1] for row in data]

will raise an IndexError, because the trailing newline row will not have second element:

>>> data = 'a,b\nc,d\n'.split('\n')
>>> print(data)
['a,b', 'c,d', '']
>>> print(data[0].split(','))
['a', 'b']
>>> print(data[-1].split(','))
['']

Defend against this by testing that the row is not an empty string before splitting the values:

x = [row.split(',')[0] for row in data if row]
y = [row.split(',')[1] for row in data if row]

You also need to remove the column header names from the the values that you are passing to matplotlib. Do this by omitting the first row when creating the x and y values:

>>> data = 'First,Second\na,b\nc,d\n'.split('\n')
>>> print(data)
['First,Second', 'a,b', 'c,d', '']
>>> x = [row.split(',')[0] for row in data[1:] if row]
>>> print(x)
['a', 'c']
>>> y = [row.split(',')[1] for row in data[1:] if row]
>>> print(y)
['b', 'd']
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8 Comments

Thanks for the help! however when I try with the ` x=[row.split( ' , ' ) [ 0] for row in data]` I get the below: IndexError : List index out of range....?
@AlexWalton are you sure it's on the x = ... line and not the y = ... line? The str.split method always contains at least one element in my experience, so an IndexError for index 0 seems unlikely. But an IndexError on index 1 could happen if your file contains a line without a comma - for example if there's an empty line at the end.
Thanks for coming back! Ok, I wasn't formatting the code correctly. Yep, you're correct, it is on the y = ... and not x = ... . I went back to the original text file and tried to removed and "spaces" but it still causes the error. So what its actually doing is getting confused with the empty line at the end (or if there is a line without an error!)
Does y = [row.split(',')[1] for row in data if row] work?
It allows me to go further into the code, but throws up another error: return array(a, dtype, copy=False, order=order) ValueError: could not convert string to float: Value
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