linear

The identity coordinate transform: Cartesian x and y, unchanged. It is what a chart uses when you pass no coord, so you only name it explicitly to override an enclosing coordinate space back to Cartesian.

Not in the Python API yet

The Python wrapper exposes polar(), clock(), and wavy(); linear() has no Python factory, so a Python chart gets Cartesian coordinates by leaving coord off. See the JavaScript page for the explicit spelling.