Y axis not defined by data

When plotting two graphs in panels, the y-axis minimum and maximum is the same in both graphs, even though the online help says
```
-ymin YMIN, --ymin YMIN
Minimum value of Y-axis. Default = defined by data.
-ymax YMAX, --ymax YMAX
Maximum value of Y-axis. Default = defined by data.
```
this can obviously be interpreted in different ways, but it would be nice to have the choice to have different Y-axes for two plots. So not clear really whether this is a bug or an enhancement request.

Here is the command line used with the attached file
```
plotxvg -f minus.xvg plus.xvg -panel -yframe 16 -legend_x 0.8 -xmax 3 -sfx -0.1 -sfy 0.96 -sharelabel
```

[mp.zip](https://github.com/user-attachments/files/24072509/mp.zip)