having obtained the amplitude (in milli volts y -axis) versus the time (in seconds x - axis). That is how does one conduct statistical analysis on heart rate variability data, from 6-12 participants.
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I'd start with the current standard ‘Heart Rate Variability: Standards of Measurement, Physiological Interpretation, and Clinical Use’, Circulation (1996). It suggests the Burg algorithm. You might also find ‘Power Spectral Density of Unevenly Sampled Data by Least-Square Analysis: Performance and Application to Heart Rate Signals‘ IEEE Trans. Biomed. Engr. (1998) and ‘On the quantification of heart rate variability spectral parameters using time–frequency and time-varying methods’ Phil. Trans. Royal Society A (2009) helpful. As you can see, these are all established techniques.
The statistics you do after that depend on your experimental design. I sincerely hope you designed your study with your hypotheses and the eventual statistical analysis of your results in mind before you collected your data rather than the other way around.