No AC voltage shows a high AC frequency (Hz)

I recorded the transition from mains power to generator power and back again. During the 20 seconds while the power is off, the IoTaWatt recorded the AC frequency (Hz) to be 1652Hz with zero voltage?? So, I must zoom in on the chart to see the AC frequency transition from mains to generator and back again. The generator AC frequency bounces around with AC voltage movement, while the mains AC frequency is very stable. I wanted to show this on a chart, but the erroneous 1652Hz frequency prevents it. Is there a way to display a zero AC frequency when the voltage reference is zero?

The chart shows the following:
mains power -> off power for 20 seconds -> generator power -> mains power is back

Here’s a zoom in on the chart showing the transition from generator power to mains power:

I worked around this issue by setting a maximum value for the Y-axis on the chart. However, I thought it strange that zero volts could have a non-zero frequency.