Fix Python – Change values on matplotlib imshow() graph axis

Question

Asked By – Lee

Say I have some input data:

data = np.random.normal(loc=100,scale=10,size=(500,1,32))
hist = np.ones((32,20)) # initialise hist
for z in range(32):
    hist[z],edges = np.histogram(data[:,0,z],bins=np.arange(80,122,2))

I can plot it using imshow():

plt.imshow(hist,cmap='Reds')

getting:

enter image description here

However, the x-axis values do not match the input data (i.e. mean of 100, range from 80 to 122). Therefore, I’d like to change the x-axis to show the values in edges.

I have tried:

ax = plt.gca()
ax.set_xlabel([80,122]) # range of values in edges
...
# this shifts the plot so that nothing is visible

and

ax.set_xticklabels(edges)
...
# this labels the axis but does not centre around the mean:

enter image description here

Any ideas on how I can change the axis values to reflect the input data I am using?

Now we will see solution for issue: Change values on matplotlib imshow() graph axis


Answer

I would try to avoid changing the xticklabels if possible, otherwise it can get very confusing if you for example overplot your histogram with additional data.

Defining the range of your grid is probably the best and with imshow it can be done by adding the extent keyword. This way the axes gets adjusted automatically. If you want to change the labels i would use set_xticks with perhaps some formatter. Altering the labels directly should be the last resort.

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

ax.imshow(hist, cmap=plt.cm.Reds, interpolation='none', extent=[80,120,32,0])
ax.set_aspect(2) # you may also use am.imshow(..., aspect="auto") to restore the aspect ratio

enter image description here

This question is answered By – Rutger Kassies

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