Unable to save an image from a graph with MAVExplorer

Open a tlog, select the predefined graph for gyro comparison, select the save icon, and the following error happens:

```
Traceback (most recent call last):
File "/home/wickedshell/.local/lib/python3.10/site-packages/matplotlib/backends/backend_wx.py", line 1156, in save_figure
dlg = wx.FileDialog(
wx._core.wxAssertionError: C++ assertion "volDummy.empty() && pathDummy.empty()" failed at /tmp/pip-install-5sm5wn88/wxpython_1da81c53a7384dbf978213a3e558d6f8/ext/wxWidgets/src/common/filename.cpp(544) in Assign(): the file name shouldn't contain the path
```

System info:
``` shell
[wickedshell@lackey ~]$ pip show wxpython
Name: wxPython
Version: 4.1.1
Summary: Cross platform GUI toolkit for Python, "Phoenix" version
Home-page: http://wxPython.org/
Author: Robin Dunn
Author-email: robin@alldunn.com
License: wxWindows Library License (https://opensource.org/licenses/wxwindows.php)
Location: /home/wickedshell/.local/lib/python3.10/site-packages
Requires: pillow, six, numpy
Required-by:
[wickedshell@lackey ~]$ python --version
Python 3.10.1
```

Start by reproducing the failure in MAVExplorer with a tlog and the predefined gyro-comparison graph, then follow the save action into the matplotlib wx backend shown in the traceback. Done means the save dialog opens without the wx assertion and the graph image can be written successfully.

Written by the indexing model from the issue text.