The best documentation for the serial data sent by the cheapstat is probably the firmware source. You can download it from here http://web.chem.ucsb.edu/~kwp/cheapstat/
I've also but a copy of the firmware in a repository on bitbucket which includes a makefile for building and uploading the fimrware on linux https://bitbucket.org/iorodeo/cheapstat_firmware/src
Note, we are not the original authors of the cheapstat firmware, but I have some familiarity with it. In the cheapstat.c file each test has its own function. For example for the CV test the function is called "CV_test". The serial data is sent at the bottom of this function. As an example, in the "CV_test" function look at the code following the "//start output to USB" comment.