The purple dot means you have logged a symptom on this day.
Here's how to see your logs for a particular day:
1. Navigate to the calendar. Tap or pull up the mini upwards arrow near the mid-bottom of the screen to reveal the text area to see everything you have logged.
2. Navigate to the logging button (the + sign at the bottom of the screen), then select the desired date. The categories in which you have logged a symptom are highlighted with a green checkmark.