Matplotlib graphs data wrong from two lists of x and y points

Question

These are the two lists:

x = [736297.48567129625, 736297.53253472224, 736297.57858796301, 736297.50894675928, 736297.55557870376]

y = [-757, -706, 385, -247, 126]

Here's my code:

import matplotlib.pyplot as plt
import matplotlib.dates as mdate

x = [736297.48567129625, 736297.53253472224, 736297.57858796301, 736297.50894675928, 736297.55557870376]
y = [-757, -706, 385, -247, 126]

plt.figure()
plt.plot_date(x, y, fmt='b-')
plt.grid(True)
plt.show()

The x list values are converted from epoch time, using mdate.epoch2num(epoch).

This is the graph I get, which is obviously not plotted chronologically:

Matplotlib improperly plotted chart

Bonus question: You can see that the time format on the x axis looks horrible. Any idea how to present it in the format yy-mm-dd-hh-ss?

Thanks to all python gurus for your assistance!

pltrdy · Accepted Answer · 2016-12-12 16:09:08Z

1

pyplot is just plotting in the order you provided your data. It turns out that x isn't sorted so the plot kinda goes "backward".

You should then sort you indices (and its corresponding values) to get what you want:

#Sort by date (x) and apply change to values (y) 
xx, yy = zip(*sorted(zip(x, y)))
plt.plot_date(xx, yy, fmt='b-')

Giving you the following plot:

pltrdy

Bonus question: Have look here for pyplot dates tick formatting. It may be easy to understand and do what you want.

edited Dec 12, 2016 at 16:09

answered Dec 12, 2016 at 15:57

pltrdy

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2 Comments

Hex Over a year ago

Excellent, thank you! Since I'm obtaining these two lists from a dictionary, do you think it will be faster to get the data as [(time, value), (time, value)] and then sort the list, before plotting, versus zipping?

pltrdy Over a year ago

Not sure to understand your point. If you don't have hugely huge data this may be ok anyway. I usually used library like Pandas or numpy so I don't really what is the best when built in function are involved (because they are not, usually, the best if performance matters)

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