DSP Concepts Visually Explained
I don't know if it qualifies as quite "visually stunning", but you might want to check out my blog article:  DFT Graphical Interpretation: Centroids of Weighted Roots of Unity

The concept of the $1/N$ normalized DFT as a center of mass calculation was a major "aha moment" for me.  It is a good answer for "What does the DFT really mean?"



By request, here is one of the figures from my article:



A little explanation is in order.  The top graph is a time domain representation and the polar graphs on the bottom are on the complex plane.  The left most circle is for bin zero, aka the DC bin, the second is bin one, and so on.  The little blue circle is the center of mass and is also the bin value as as complex number.

$$ A = \frac{2 \pi n}{N} $$

This sample has 3 cycles per frame with a phase of 3.  Bin three (the fourth polar graph) clearly shows the bin value has magnitude of $1/2$ and the phase value of 3 is almost $\pi$ and therefore almost halfway around the circle.

There are many more examples, and more thorough explanations with the math in the article.