Aquila DSP C++ Library - Wave file FFT analog frequency off by factor of 4?
Not familiar with Aquila in particular, and I don't see how you're mapping the output spectrum samples to an X-axis that ends at 800 Hz,  but remember that a real-only FFT mirrors its output data in the second half of the buffer.  You may need to run your loop from 0 to window_size/2 if Aquila's FFT is implemented the usual way.  

That could account for a factor of 2x.