Understanding MySQL Slow Query graph on Grafana

I’m having troubles understanding what the graph shows for the MySQL Slow Query graph, particularly with what the y axis represents.
I’ve attached photos of some results I see, I understand the graph shows all slow queries that took longer than the long_query_time variable set. For my case the value is 0.25 . So I’m seeing all queries that took longer than 0.25s. But I thought I would be seeing number i.e. integers in the y axis to show number x slow queries within a specific time frame. But i’m seeing decimals, lower than 0.25 value which confuses me.

Usually values ranges between 0 and 1 but some cases spikes very high to 32,000.

Does anyone know how to interpret this graph? I’m trying to set alert notifications but won’t know suitable numbers to choose unless I fully understand what graph represents.