The spectral analysis parameters calculated by LabChart's HRV module (Total, HF, LF etc) are based on the RR interval signal, which is generated by the beat detector first analysing the raw data. These RR intervals are typically measured in ms. The mathematical transform used to generate the power spectrum results in a squaring of the input data's unit. Hence a unit of ms^2 is expected.
These units are correctly displayed in HRV Module version 1.x (for Windows and Mac) and in version 2.0.3 and above (for Windows) and version 2.0.4 and above (for Mac).
Versions 2.0.0 to 2.0.2 (for Windows) and versions 2.0.0 to 2.0.3 (for Mac) displayed the unit as µs^2. The value of the results was identical to all other versions as µs^2 was interpreted as µ(s^2), which is equivalent to ms^2 e.g. 1 (ms)^2 = 0.001 s x 0.001 s = 1e-6 s^2 = 1 µs^2, or 1 µ(s^2).