Just as a datapoint for Tracy.

The way I do it is almost too simple. I just output each "channel" in sequence, each channel separated by the Tab character (9), and each line (or data set) separated by CrLf (13+10). No labels, just the raw numbers.
Requires a very simple program to capture serial input on the PDA or laptop and write to a text (.txt) file.
The resulting text file can be opened directly with Excel and you just need to add the headings row. The graphing features in Excel are very easy to use.

As for publishing the data format, all that would be needed is a description of what is in each channel and units used.

I forgot, the first "channel" is time. Example of one log line:
12:49:24 PM    5115     6.0     7.2     96    106     13     62     88     2.3    910    894     0.0    21.6     26    00    00    17    74    13.8               

Speadsheet heading row:
Time    RPM    GPH    GalsUsed    WT: Water Temp (C)    LRT: Air temp after left radiator (C)    RRT: Air temp after right radiator (C)    OT: Oil temp (C) after oil cooler    OPT: Oil pan temp (C)    WP: Water pressure (psi)    OP: Oil pressure (psi)    OFP: Fuel pressue two outer carbs (psi)    CFP: Fuel pressure center carb (psi)    EGT1 (C) Rotor 1: nearest water pump    EGT2 (C) nearest flywheel    MAP (inHg)    Mix: Mixture: O2 sensor (mV)    OAT: Outside Air Temp (C) - defect    Volts (system voltage)

Finn