Jason Stone, on 2011-02-18 14:39, wrote:

Good afternoon all,

```
> One last matplotlib question for the group for today. On
```

one of my GUI

```
> plots, I'm calling imshow on an array of data (to display
```

it in the same way

```
> MATLAB's imagesc command does). I'd like to add a second
```

y-axis to the

```
> right side of the plot that is completely dependent on the
```

values on the

```
> primary y-axis. Essentially, for each y-axis tick point,
```

I’ll put the

```
> y-axis 'value' into a formula and then put the result on
```

the second y-axis.

```
> I did this in MATLAB by essentially overlaying a second
```

set of axes over

```
> the plot, but I haven't found the exact way to do it with
```

matplotlib yet.

```
> I've seen a few examples online, but they all use the
```

second overlaid plot

```
> to actually plot new data - I wouldn't be doing this.
> Would I need to use the twinx (or twiny) function?
> Are there examples of this on the web that I haven't found
```

that somebody

```
> could point me towards?
```

Hi Jason,

```
here's an example that does what you want, using e^x as the
formula, change the paramter to fmtr to suit your needs:
ax = plt.subplot(1,1,1)
ax.plot(np.sin(np.linspace(0,np.pi)))
ax2 = ax.twinx()
ax2._sharey = ax # share both x and y
fmtr = mpl.ticker.FuncFormatter(lambda x,pos: "%.2f"%np.exp(x))
ax2.yaxis.set_major_formatter(fmtr)
plt.draw()
best,
```

## ···

–

```
Paul Ivanov
314 address only used for lists, off-list direct email at:
[http://pirsquared.org](http://pirsquared.org) | GPG/PGP key id:
```

0x0F3E28F7

```
Paul,
I am currently doing something very similar and was hoping I could
```

ask for a little clarification. I want to have two y axes where the

ticks are in the same locations and the 2nd y-axis labels are just

twice the 1st y-axis labels. When I implement your example, I don’t

quite understand how to ensure the ticks are at the same locations.

Also, when the data changes to a new image, the 1st y-axis updates

but the 2nd y-axis does not. Is there a convenient way to force both

to update each time, or does the figure need to be cleared and

essentially built from scratch each time.

```
for example, with the following code:
import numpy as np
import matplotlib
import matplotlib.colorbar as cb
import matplotlib.pyplot as plt
y = np.reshape(np.arange(0, 1000000, 1), (20000, 50))
test = 'hot'
f1 = plt.figure(1)
f1.patch.set_facecolor('#c0c0c0')
ax1 = f1.add_axes([0.09, 0.15, 0.82, 0.80])
axc = f1.add_axes([0.09, 0.05, 0.82, 0.05])
im1 = ax1.imshow(y, cmap=test, aspect='auto', origin='lower')
cb.Colorbar(axc, im1, orientation='horizontal')
ax2 = ax1.twinx()
ax2._sharey = ax1 # share both x and y
fmtr = matplotlib.ticker.FuncFormatter(lambda x,pos:
```

“%.2f”%np.exp(x))

```
ax2.yaxis.set_major_formatter(fmtr)
plt.show()
The ticks on the right are obviously at different locations compared
```

to the left ticks. Also, if imshow was to be called a new array of

data, the ticks on the right would remain the same. So is there an

easy way to force the locations to be the same, and is there an easy

way force the right y-axis to update each time? Thank you very much

for your time and help.

```
-Thomas
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