What's df (frequency resolution)?
With Fmax=1 MHz, it would requre 10,000 points in the FFT to have a 10 Hz resolution, possible but 0.1 Hz would be 100,000 points minimum.
From the plot, looks like the maximum is almost certainly in the DC bin; did you remove the mean before taking FFT?
In short, to answer the Q? need to know what were the sampling parameters
Altho, presuming the frequency axis is correct, then the indices for the bins are
idx=[0.4 10]/df+1; % +1 accounts for f(1)--> 0 Hz (DC component)
where df is the frequency resolution.
