Astro.com has biorhythm chart calculations available under the extended charts or special charts headings; I believe these indications are worthwhile can be used to plot when certain astrological correlated influences are actually having a definite impact upon one's biofield (not every indicated astrological influence, such as a transit, will actually TAKE upon the person when it occurs-biorhthym can be a tool to help in this regard)
Funny thing-and I currently believe this was just a coincidence-but I checked the biorhythm for the day of the Haiti earthquake on astro.com-the figure was extremely activated for that day-much moreso than for any date in the projected year in the "Haiti biorthym" chart! How can Haiti possibly have a biorthym? I did the same for the Chile quake-result was not as intense for the "Chile" biorthym chart as for the Haiti chart, but it was more than a little similar to it in intensity! I have not followed up these investigations, and I find it hard to accept they were anything more than coincidence, but then who knows? Maybe what we have termed biorthym is more, and fundamentally different in its true nature, than what we believe it to be?