How to show delta on interval

I have an metric - total count of added messages. And I want to see on graph delta per this time, not absolute values.
For example, it has values 100 010 and 100 020 values per the interval. So, I want to see graph from 0 to 10, just delta per period, because change of 10 is unseen on just value graph.
I am absolutely dummy in Grafana, help me please.