In a few words, biorhythm is an hypothetical cycle that affects humans in some aspects of life. Each aspect (physical, emotional, intellectual) has a different period (23, 28, 33 days respectively), and all periods begin when we are born, so the cycles will diverge and reconverge from time to time.
People should watch out for "negative" phases of each period, and the biorhythm "theory" stresses that crossing days (when a cycle transitions from negative to positive and vice-versa) are the most dangerous.
Even more stressful are double-crossing or triple-crossing days, that is, when two or three cycles transition in the same day. Some say that suicide rates correlate strongly with multiple crossings, as well as with triple-bottom days (when all three cycles are in the bottom, or around it).
Some calculators, including this one, add a apocryphal "Total" curve, which is simply an average of the canonical three curves. This helps to spot general highs and lows, as well as composite zero-crossings.
Warning: the calculator assumes that you are in the same timezone that you were born. If you are not, you need to compensate for it in birth date/time. (If you can tolerate a rather small imprecision in graph, you don't need to worry about this.
License: the code of this calculator is open-source for studying purposes only. It is not allowed to duplicate it to other sites.