MatPlotLib X axis with dates

Question

I have a graph within a GUI which opens with a button click. It opens from a csv file that constantly updates, hence the counting at the beginning, I wanted to limit the number of entries for the x-axis.

The plot works for the four lines however, I have been unable to refine the x axis, what I would like to do is;

Rotate the font
Remove the trailing zeros for the dates
Possibly Change the Date Format to D-M-Y H:M

The Below script is what I have so far:

def open_graph():
    import numpy as np
    import matplotlib.pyplot as plt
    import datetime as dt 
    from dateutil import parser
    from csv import reader
    with open('Voltage_Readings.csv','r') as f:
        data = list(reader(f))

    file=open('Voltage_Readings.csv')
    numlines=(len(file.readlines())-1)
    start=numlines-100
    end=numlines

    fig=plt.figure()
    ax=plt.subplot(111)

    DTG = [parser.parse(i[1]) for i in data[start:end]]
    Battery_Level = [i[2] for i in data[start:end]]
    Gene_Supply = [i[3] for i in data[start:end]]
    Solar_Supply = [i[4] for i in data[start:end]]
    Wind_Supply = [i[5] for i in data[start:end]]

    plt.plot(DTG, Battery_Level,'-r', label='Battery')
    plt.plot(DTG, Gene_Supply,'-y', label='Gene')
    plt.plot(DTG, Solar_Supply,'-b', label='Solar')
    plt.plot(DTG, Wind_Supply,'-g', label='Wind')

    box = ax.get_position()
    ax.set_position([box.x0, box.y0, box.width *0.85, box.height])
    ax.legend(bbox_to_anchor=(1, 1),loc=2)

    plt.title('VOLTAGE MEASUREMENTS FROM POWER SOURCES')
    plt.xlabel('Time')
    plt.ylabel('Voltage')

    plt.show()

I would appreciate any assistance is being able to achieve this as I am still learning.

Bobby Durrett · Accepted Answer · 2017-02-16 16:46:41Z

1

Here is a better answer:

Just add this before your plt.show() command:

fig.autofmt_xdate()
import matplotlib.dates as mdates
ax.fmt_xdata = mdates.DateFormatter('%d-%m-%Y %H:%M')
datetimefmt = mdates.DateFormatter("%d-%m-%Y %H:%M")
ax.xaxis.set_major_formatter(datetimefmt)

Running autofmt_xdate makes the dates slanted. Setting ax.fmt_xdata sets the date format when you hover your mouse over a point. Running ax.xaxis.set_major_formatter sets the date format on the x axis.

Interestingly enough in my own matplotlib code I've done my own formatting of the dates on the x axis because I wasn't aware of these auto formatting features. I'm going to have to revise my code to use what's already in matplotlib.

Bobby

edited Feb 16, 2017 at 16:46

answered Feb 9, 2017 at 21:31

Bobby Durrett

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3 Comments

user7524882 Over a year ago

@bobbydurrettdba, thanks for the advice, however I cant seem to get it to work. My script which is reading from csv doesn't appear to like that line. If you wouldnt mind could you look at my csript and let me know where it needs to go or if I need to add anything else. I am also not too sure on how to configure xnames as a date in the format I want.

Bobby Durrett Over a year ago

Sorry, I wasn't very clear. It looks like DTG is a list of dates and times in your code. In my code my list of dates and times is in the variable named xnames. I think if you want to change your dates just make a new list from DTG modifying each element to be the length that you want and the format that you want. xticks takes rotatation=angle as an argument and that is how I rotated my dates along my x axis. The first argument to xticks is a list of x values of the labels and the second argument is a list of labels.

Bobby Durrett Over a year ago

I edited my answer. Matplotlib has some nice built in date formatting that I was not aware of before this.

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