Ocavis visualises the intonation of notes played on your ocarina in relation to the pitches of equal tempered notes over time.
Ocavis can be used for a number of things:
Ocavis is very useful for checking that the breath curve you are using is correct for your ocarina. Simply play a scale slowly from the lowest to the highest note, and look at how the notes fall on the graph.
You should see something like the following, where the lines drawn from your playing have the same spacing as the lines in the background.
If your blowing is too shallow, the pitches of your notes shown in Ocavis will be closer together than the background lines.
If your blowing is to steep, your pitches will be further apart than the background lines.
You may find that the pitches shown by Ocavis are correct for most of the range, but are incorrect on a few notes. This may be as your ocarina requires a non-linier pressure curve. Many 12 hole ocarinas for example require an exponential pressure spike on the highest 3 notes for them to play in tune.
If you play a simple scale like this and your pitches appear random, either you are changing your blowing pressure irregularly, or the ocarina you have is badly tuned.