Pick one of the radio buttons "Region A" or "Region B" and draw the region with the mouse in the canvas.
Each region will highlight a set of outcomes (represented by the small circles inside the canvas). Region A will represent the event "A" and region B, the event "B". The outcomes inside each region will be the favorable outcomes for each event.
Pick the form of Bayes formula you want to visualize in the selection box.
$\#A$, $\#B$, $\#AB$ and $N$ will be number of favorable outcomes for region A, B, A intersection B and total respectively.
$P(A)$, $P(B)$, $P(AB)$ and $P(\bar{A})$ will be the probabilities of the event A, B, AB and the complement of A respectively. $P(A|B)$ and $P(B|A)$ are the posteriori (conditional) probabilities.

When you hover over an element in the formula, it highlights the outcomes involved in the formula.