Some issues about ingest/kafka/lag

I had put the druid metric ingest/kafka/lag into grafana, one of datasources is below:

I found it is not equals to 0 mostly, but we have never found the data delay in our datasources. How can we understand that?

and btw, how do we know the data has been delayed or not through the picture?

the picture has been broken, I had attached the file, thank you!

It doesn’t work, the picture has still been broken.