While blue whales belt out calls for hours on end at levels up to 190 dB underwater that can be heard at distances up to hundreds of kilometers, gray whales are just the opposite. Worse yet, it is hard to distinguish some of their calls from those of fish or other marine animals. This is one reason why having JJ at Sea World was so important. The vertical axis shows frequency - roughly, pitch. Low frequencies are low pitched, shown on the lower end of the scale, whereas high frequencies are high pitched, shown on the upper end of the scale.
Time is shown on the horizontal axis.
You can see from the pictures here that both wild whales and JJ have most of the energy in their calls at low pitch. Based on these pictures, JJ's vocalizations could be compared to ones made by wild gray whales. She didn't seem to make the 'knocks' or 'clacks' that were so familiar in recordings of wild whales, though. The playbacks started 2 minutes before her feedings so she would learn to associate the sounds with food, in the hope that she would learn to pay attention to them, and that she would associate gray whale calls with feeding. Ultimately, this would help her find other gray whales when reintroduced, even if she never learned the calls.