Wrong values in the aggregation chart

Hello community.
I have saved for some time for each day the power consumption of a device and is also persisted.
I can evaluate the data per day with a chart.

Now I would like to have an aggregation per month.

However, if I set sum of my item per month, then does not come the correct result.
e.g. Oct: Sum of single values is 15,05 (all values under 1) but in the chart for sum of October it says 1.446,668.

What am I doing wrong?

Addendum: if I choose average or maximum in the aggregation, the correct values of the month come out.