Axes are lines containing units of measure in a specified scale. Every axis is connected to a scale, which should be the same scale that's used for the data points it represents. D3 generates SVG for horizontal and vertical axes using the methods in the d3-axis module. Each axis consists of an SVG <path> element (of the domain class), followed by several <g> elements (of the tick class), each containing a small <line> (perpendicular to the path line) and a <text> element. There are methods to configure the positions, size, and padding of the tick marks, and you can change any position and style using D3 selection operations or CSS to create any axis-based system you like.
An axis is created using one of the four generator functions listed in the following table, which require a scale (that can also be provided later using the scale() configuration...