Vertical axis in "Network Connections in the Last 48 Hours"

In my use-case I have two devices close to each other that show different behaviours in the chart for “Network Connections in the Last 48 Hours” on hologram dashboard under “Devices”:
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and
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The first of the two devices is a bit unreliable with sending data and there is obviously a difference. However, the vertical axis in this plot is not specified. For the purpose of trouble shooting the first device: Does anyone know the meaning of the vertical axis in this chart?