Show Difference per Interval (group by?)


I have an incrementing value it gets bigger with time. I would now like to output daily or monthly the increase in the interval.

01.01.2020 200
01.02.2020 500
01.03.2020 900

January then shows 300
February then shows 400

The respective difference to the previous month should now be displayed in a “dynamic” diagram or as values in a table.

Unfortunately I do not get it right and hope for your help. Many thanks in advance