First figure is the input signal ($x_1(n) = \sin(2\pi f_0nt_s)$, $f_0$ frequency, $t_s$ being the sampling period) at 1Hz. Second is the input signal squared ($y_1(n) = \sin(2\pi f_0nt_s) \cdot \sin(2\pi f_0nt_s)$). Third figure is the second signal displayed in frequency domain. How can it be that the amplitude is $-0.5$ when the second figure is strictly positive and the amplitude is $1$?
Note: I am a complete beginner at this.