I had seen one or two charts available at a cost on the Internet but this free access one is definitely the best I have seen so far.
Although designed by a Lyme Disease sufferer I can recognise the value of it for many other Chronically ill patients and in particular those with Myalgic Encephalomyelitis or Chronic Fatigue Syndrome ME/CFS.
I am assured that there will be no data stored other than the e mail addresses for the log in and that can be as fictitious as you choose.
One tip - if after many months a four weekly pattern emerges, that is typical of Lyme Disease so if you haven't considered Lyme as a differential diagnosis then seek a good Lyme Literate Medical Doctor's opinion. Lyme symptoms cycle about four weekly on treatment but also when not on treatment although not everyone is aware of these cycles and when we are at our sickest it is particularly difficult to see a pattern.
Here is the link to the Symptom Grapher here