It is essential to separate the concept from the diagram, because they are separated by well over a thousand years.
Yin and yang as a pair of complementary, alternating, mutually generating aspects is argued for in Chinese texts of the Warring States period and is fully developed in the commentarial layers attached to the Book of Changes, particularly the Great Commentary, and in the correlative cosmology of the Han. The core claims are that the two are not opposed forces but phases of one process, that each contains the seed of the other, and that neither is good or evil. Yin is the shaded side of a hill, yang the sunlit side; the metaphor is topographical and the terms are relative, so the same thing is yin in one relation and yang in another.
None of this required a picture, and for centuries there was not one.
The Diagram of the Supreme Polarity produced by Zhou Dunyi in the eleventh century is the famous early chart, and it is genuinely important: its accompanying explanation became a foundational text of Neo-Confucian metaphysics, tracing the generation of the five phases and the ten thousand things from the supreme polarity. But Zhou Dunyi's diagram is a vertical stack of concentric circles and half-shaded bands. It does not look remotely like the symbol now called by that name, and reproductions captioning the modern swirl with his name are simply wrong.