Python - matplotlib, pyplot, x and y errorbars the wrong way round?

Question

I am confused by the order of arguments to plt.errorbar

I have the following code in my Python script

plt.errorbar(0.0, 1.0, 0.1, 10.0)

I would assume the order of arguments is: x_data, y_data, x_error, y_error

This agrees with: http://matplotlib.org/1.2.1/examples/pylab_examples/errorbar_demo.html

However, when I run this code I get the following output:

Clearly, x=0.0, y=1.0, x_err=10.0, y_err=0.1

So the x_error and y_error arguments are swapped!

My question is why? Is the documentation wrong? I am so confused!

tmdavison · Accepted Answer · 2016-10-20 14:45:21Z

5

If you don't use keywords, the order of the arguments is x, y, yerr, xerr.

From the docs:

Call signature:

errorbar(x, y, yerr=None, xerr=None,
     fmt='', ecolor=None, elinewidth=None, capsize=None,
     barsabove=False, lolims=False, uplims=False,
     xlolims=False, xuplims=False, errorevery=1,
     capthick=None)

To avoid this, you can use the keywords to make sure you are giving the correct value to the correct argument; then the order doesn't matter. Note that this is how the example you linked to does it too.

So, for your example, you would want to use:

plt.errorbar(0.0, 1.0, xerr=0.1, yerr=10.0)

answered Oct 20, 2016 at 14:45

tmdavison

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2 Comments

user2138149 Over a year ago

Thanks - I didn't know I could re-order arguments in Python!

Paul H Over a year ago

@user3728501 you have to name them if you want to reorder them. (In my opinion, you should always name them)

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