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Warning: I'm very new to using python.

I'm trying to graph data using error bars but my data has different values for the error above and below the bar, i.e. 2+.75,2-.32. 

import numpy as np
import matplotlib.pyplot as plt

# example data
x = (1,2,3,4)
y = (1,2,3,4)

# example variable error bar values
yerr = 0.2

plt.figure()
plt.errorbar(x, y, yerr,"r^")
plt.show()

But I want the error bar above the point to be a specific value like .17 and below the point to be a specific point like .3 Does anyone know how to do this?

Thanks!

xnx
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Rima
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1 Answers1

9

Like this:

plt.errorbar(x, y, np.array([[0.3]*len(x), [0.17]*len(x)]), fmt='r^')

Pass an array of shape (2,n) with the -errors on the first row and the +errors on the second.

enter image description here

(Note also that you need to explicitly pass your format string r^ to the fmt argument).

If you want different error bars on each point, you can pass them in this (2,n) array. You typically have a list of -err and +err value pairs for each data point in order, in which case you can build the necessary array as the transpose of this list:

yerr = np.array([(0.5,0.5), (0.3,0.17), (0.1, 0.3), (0.1,0.3)]).T
plt.errorbar(x, y, yerr, fmt='r^')
plt.show()

enter image description here

This is described (more-or-less) in the docs.

xnx
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  • Thanks for answering! I have one more quick question. If each point has a different error bar value, what would I do? Like if the first point has values +.17 and -.3 but the second point in my data set has +.8 and -.23. – Rima Jun 26 '15 at 22:48
  • You use an array of shape (2,n) where each of the n pairs of values is the (-err, +err) for a point. – xnx Jun 27 '15 at 04:28