# formatting colorbar in topomap

**URL:** <https://mne.discourse.group/t/formatting-colorbar-in-topomap/6949>\
**Category:** Support & Discussions\
**Tags:** visualization\
**Created:** [May 23, 2023, 3:14pm UTC](https://mne.discourse.group/t/formatting-colorbar-in-topomap/6949 "2023-05-23T15:14:19Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![anapesq](https://avatars.discourse-cdn.com/v4/letter/a/67e7ee/32.png) [@anapesq](https://mne.discourse.group/u/anapesq)\
**Post date:** [May 23, 2023, 3:14pm UTC](https://mne.discourse.group/t/formatting-colorbar-in-topomap/6949/1 "2023-05-23T15:14:19Z")

</div>

Hello,

I am using mne.viz.plot\_evoked\_topomap and would like the colorbar to show only 3 ticks (the min, the middle and the max)

It seems like the argument cbar\_fmt in the plot\_evoked\_topomap could permit this.  
Does anyone has an example using the cbar\_fmt ?

**cbar\_fmt** [`str`](https://docs.python.org/3/library/stdtypes.html#str)

Formatting string for colorbar tick labels. See [Format Specification Mini-Language](https://docs.python.org/3/library/string.html#formatspec) for details

Many thanks!

Ana

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<div class="post-metadata">

**Author:** ![Hugo-W](https://yyz2.discourse-cdn.com/free1/user_avatar/mne.discourse.group/hugo-w/32/2250_2.png) [@Hugo-W](https://mne.discourse.group/u/Hugo-W)\
**Post date:** [May 24, 2023, 12:40pm UTC](https://mne.discourse.group/t/formatting-colorbar-in-topomap/6949/2 "2023-05-24T12:40:52Z")

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Hi,

This may not be MNE-related, really… I don’t think the functions of MNE expose this colorbar property.

The **cbar\_fmt** you mentioned is to format the number shown at each tick into a specific string (how many digits after the point, or leading zeros, or scientific number with exponents, etc…).

To alter the tick **locations** , you can pass the ticks you want to the `colorbar` call when you create it, e.g. `plt.colorbar(im, cax=cax, ticks=[np.min(data), np.median(data), np.max(data)])`. And more generally, you can use [Locators (see Matplotlib’s documentation)](https://matplotlib.org/stable/api/ticker_api.html#tick-locating-and-formatting) to alter the position of ticks.

In my opinion, it is much easier to create your own colorbar rather than altering the one that MNE outputs with `plot_evoked_topomap`.

Here is an example with either a colorbar added by “hand” to the final figure or by modifying properties of the existing colorbar:

```python
import mne
import numpy as np
import matplotlib.pyplot as plt

own_cbar = True # switch between creating a colorbar or using the one from the plot_topomap call

# Random data
montage = mne.channels.montage.make_standard_montage('biosemi64')
info = mne.create_info(montage.ch_names, 100, 'eeg')
info.set_montage(montage)
evoked = mne.EvokedArray(np.random.randn(100, 64).T * 1e-6, info)

# Create figures with 3 topographies
fig = mne.viz.plot_evoked_topomap(evoked, times=[0.1, 0.25, 0.5], colorbar=not own_cbar, show=False);
im = fig.axes[-1 if own_cbar else -2].images[-1] # select the last image to grab/create colorbar
if own_cbar:
    cax = fig.axes[-1].inset_axes([1.15, 0.05, 0.075, 0.9])
    cb = fig.colorbar(im, cax=cax)
    cax.set_title(r'$\mu$V')
else:
    cb = im.colorbar
# The locator is what defines the tick on the colorbar object in matplotlib    
cb.locator = plt.LinearLocator(numticks=3)
cb.update_ticks()
plt.show()

```

![example_cbar](https://global.discourse-cdn.com/free1/uploads/mne/original/2X/d/d3a9743187529c3503a0e221573acee3bd2297e5.png)

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<div class="post-metadata">

**Author:** ![anapesq](https://avatars.discourse-cdn.com/v4/letter/a/67e7ee/32.png) [@anapesq](https://mne.discourse.group/u/anapesq)\
**Post date:** [May 25, 2023, 6:58am UTC](https://mne.discourse.group/t/formatting-colorbar-in-topomap/6949/3 "2023-05-25T06:58:45Z")

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@Hugo-W Thank you for this. Most useful.