I change samplingfrequency to another number also 8000
You can change the sample numbers all you like but unless the Arduino is capable of acheaving that sample rate it will do no good.
The appropriate bin will fill up based on the real sample rate you use, there is no way the calculations can compensate for a bad sample rate.
Was that graph something you produced or something you want? You have still not told us about your real application, that is what you will use that graph for.
It looks like you will not have a sufficient sample rate to be able to group the bins how you want.