ChristineB - 2 months ago 34

Python Question

**The issue**

I have a dot plot I made from imported data. One set of data shows a time series of means across various members of a dataset, and the other shows the standard deviation (SD) of these members at each time step, before the means were taken. My advisor wants me to add "whiskers" that show +/- 1 SD to the points representing the means. I'll provide a simplified, working example below.

**The code (example)**

`import numpy as np`

import matplotlib.pyplot as plt

%matplotlib inline

#Generate some data

x = np.empty((7,9))

for i in range(7):

x[i,:] = np.arange(1,10,(1+(i*.02)))

print x

#Find standard devs down each column

std = np.std(x,axis=0)*5

print std

#Get the mean of x data down each column

xmean = np.mean(x,axis=0)

print xmean

#Plot xmean data & x's stan. devs

legendlabels = ['Mean of members','S.D. of members']

time = np.arange(0,9)

fig, ax = plt.subplots(figsize=(11,6))

data1 = ax.scatter(time,xmean,s=70,color='k',marker='^')

data2 = ax.scatter(time,std, s=70,color='k')

ax.legend([data1,data2],legendlabels,loc=2)

ax.grid()

plt.show()

SD was multiplied by 5 so whiskers, when added, could actually be visible. The result should look like this.

How can I add whiskers to the triangles that represent +/- 1 corresponding SD (the values of the circles below)? Given my actual data isn't showing a range along the y-direction, I don't think a box plot would work best for this...

Answer

You could add error bars with:

```
ax.errorbar(time, xmean, yerr=std)
```

If `yerr`

is a 1D array of size `time`

, the error bars are drawn at +/- `yerr`

relative to your data (`xmean`

) (http://matplotlib.org/api/pyplot_api.html#matplotlib.pyplot.errorbar):

If a scalar number, len(N) array-like object, or an Nx1 array-like object, errorbars are drawn at +/-value relative to the data.

With your code, that results in:

For more info, see these examples: http://matplotlib.org/1.2.1/examples/pylab_examples/errorbar_demo.html