Compile and install with:
$ CC=g++ CFLAGS=-O3 gimptool-2.0 --install elsamuko-lab-analysis.c
You will find the plugin then under Colors->LAB Analysis.

This plugin uses the CImg library from David Tschumperlé for the RGBtoLab conversion:
http://cimg.sourceforge.net/

As example the famous BP image:


LAB analysis:

As you can see, the blue tones hit the borders of the RGB color space, which is a sign for tone stretching.
In comparison the analysis image of the whole RGB color space: