I had the same issue. Autochart on the Onix records & saves to the Onix memory until the memory is full. Then it records over the oldest data replacing it with the new. The only way to get the data off of the unit & save it is by purchasing an Autochart zero lines card & inserting it into the Onix. The data will be transferred from the Onix to the card. Also, you can record directly to the card.
You will find some great information here:
Auto Chart, Auto Chart Pro & Auto Chart Live Information Resource