Python, matplotlib.pyplot cant see line plot

Question

Im trying to graph a list of tuples using matplotlib.pyplot package & Python 3 in Spyder IDE. Below is the relevant snippet of my code:

import matplotlib.pyplot as plt

def plot_test(item):
    xdata = []
    ydata = []
    for item in list_of_epoch_total_time_tuples:
        xdata = item[0]
        ydata = item[3]

    plt.plot(xdata,ydata)
    plt.ylabel('epoch_counter')
    plt.xlabel('square_error')
    plt.show()

Here is my output:

I cant see the line plot. The graph is empty?

1) How can i draw this properly

2) how can I provide plot range, so there is no negative values?

Thanks

sn8wman · Accepted Answer · 2017-12-18 02:49:21Z

First, the code from your screen grab has plt.plot(xdata,item[3:]) while the code you added to the question is plt.plot(xdata,ydata) which I assume is what you want. The problem is you're overwriting xdata and ydata on every iteration so it's just the single item value. Instead use:

for item in list_of_epoch_total_time_tuples:
   xdata.append(item[0])
   ydata.append(item[1])

This will add the value to the end of the list instead of overwriting the variables' data. Also you have item as a parameter for plot_test() but it does not appear to be used anywhere and you also use the name item as your for loop parameter. You should avoid using the same name in two places like this as it can be confusing.

For changing the range you can use:

plt.xlim(xmin=some_min, xmax=some_max)

or

plt.xlim(xmin=some_min)

if you only want to change the min. And you can do a similar command for ylim.

If you have a data set you may also want to look into using numpy. Using numpy arrays is generally faster than using lists and for loops

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