Each panel is one term of the inverse DFT: the complex exponential for frequency u, already scaled and rotated by its coefficient X[u]/M. Panels with the same colour are conjugate pairs. Tick panels on and off to see which terms the reconstruction is using.
Bars come in mirrored pairs because x[n] is real: |X[M−u]| = |X[u]|. Faded bars are terms left out of the sum. Click a bar to toggle it.