Show Difference per Interval (group by?)

Hello,

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

Example:
01.01.2020 200
01.02.2020 500
01.03.2020 900

January then shows 300
February then shows 400
etc.

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