matplotlib plot asymmetric errorbars in for loop with indexes

Question

I'm trying to plot some arrays with a for loop using plt.errorbar with asymmetric errorbars. I know that in this example it would be easier to use arrays without indexes, but I would like to use the indexes in plt.errorbar in other code with some conditions on the index i.

This is the example code:

   import numpy as np
   import matplotlib.pyplot as plt


    x=[ 0.007206,  0.043695,  0.372777,  0.464819,  0.337386,  0.249215,  0.395453,
      0.222331,  0.11715,   0.101464,  0.645596,  0.228634,  0.187252,  0.283026,
      0.596368,  0.019066,  0.300215,  0.174883,  0.331613,  0.175409,  0.858567, 0.895389]

    y=[  0.327811,  33.3177,     1.36996,   41.9717,     1.18497,    1.05182,    2.28229,
       0.424775,   1.11758,    4.5135,     2.70709,    1.26611,   10.8293,     4.92649,
      31.4483,     0.403496,   1.14471,    1.72301,   12.081,      0.501048,  13.6858,
       4.58709 ]

    xel=[ 0.034065,  0.096869,  0.046961,  0.13575,   0.086615,  0.070706,  0.068376,
      0.132277,  0.12079,   0.102303,  0.048192,  0.070823,  0.067665,  0.07266,
      0.093411,  0.040662,  0.089356,  0.098089,  0.137559,  0.146229,  0.038649,
      0.030372]
    xeu=[ 0.032612,  0.092047,  0.04424,   0.151329,  0.077828,  0.066373,  0.065701,
      0.123756,  0.09371,   0.086466,  0.047322,  0.069837,  0.070206,  0.058787,
      0.088777,  0.045837,  0.098174,  0.105,     0.148259,  0.14845,   0.133334,
      0.104611]

    for i in range(21):
        plt.errorbar(x[i], y[i], xerr=[xel[i],xeu[i]], fmt='.')

    plt.show()

The output error is:

ValueError: err must be [ scalar | N, Nx1 or 2xN array-like ]

Also If i try with symmetric errorbars (plt.errorbar(x[i], y[i], xerr=xallel[i])) it works. So I don't understand where problem is.

I need the indexes because I need to add a condition on the index i at the top of the loop. For example:

for i in range(21):
    if y[i]<1.0:
        plt.errorbar(x[i], y[i], xerr=[xel[i], xeu[i]], fmt='.', color='red')
    else:
        plt.errorbar(x[i], y[i], xerr=[xel[i], xeu[i]], fmt='.', color='b')

How can i fix that? I'm using anaconda Python 3.6.

GWW · Accepted Answer · 2017-12-16 14:03:21Z

1

Your xerr array is not 2xN, it should be 2x1:

xerr=[[xel[i]],[xeu[i]]]

You can avoid the loop as well matplotlib usually supports plotting multiple values at once.

plt.errorbar(x, y, xerr=[xel,xeu], fmt='.')

edited Dec 16, 2017 at 14:03

answered Dec 16, 2017 at 13:57

GWW

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5 Comments

Alessandro Peca Over a year ago

Yes I know i can plot all data without indexes, but I need indexes. This is a simple example for a complex problem...

GWW Over a year ago

The code above that permits you to plot without indexes

Alessandro Peca Over a year ago

I'm sorry but if I add a condition on the index, for example if y[i]<1.0, this solution plot all the points of the y array.

Alessandro Peca Over a year ago

I'm sorry, I did not see the double square brackets. now it works, thank you very much!

Alessandro Peca Over a year ago

@ImportanceOfBeingErnest there is no need to be rude. thanks also to you for your advice.

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