Bitcoin HODL Waves is a visualisation pioneered by Unchained Capital, it shows the cross section of Bitcoin held in wallets grouped by the age since they last moved. The upper contours, represent supply (old coins that have remained unmoved) while the lower contours represent new demand (coins that have recently shifted). The composite view clearly shows each bull cycle bringing in new demand. This visualisation is useful for locating exactly where the market timing is during its long term oscillations between bull and bear phases. More information is available by reading Unchained Capital`s original blog post on this subject.