[enhancement] mavgraph output to image file

It would be pretty usefull if mavgraph.py could output to a image file with a commandline option like with mavflightview.py --imagefile=IMAGEFILE.

Sometimes there are many log files/chunks to graph from testing/tuning runs. I would like to be able to automate some of the after flight analysis tasks.

thanks!
Linus

Start by reading mavgraph.py to understand how graph output is produced, then compare mavflightview.py's --imagefile=IMAGEFILE option and handling. Done means mavgraph.py accepts an equivalent command-line option and writes the generated graph to the requested image file for automated analysis.

Written by the indexing model from the issue text.