Graph is loading very slow: To many values


I have a raspberry pi3 with the standard database. Displaying the temperature of the last 2 days is no problem.

But if I want to display the chart of the power consumption it loads very long. More than a minute.
The problem is, that this chart has very much measure values. Displaying all this values is not possible, but the chart needs to calculate everything.
So my question, is there a way to reduce a measure values. May be have a second sensor which flaten/polish the values?