8.2 Surface Types

The different plot2* commands correspond to different plot surface geometries. The different commands come with their own specific axis configuration parameters. Some of the plot layer types are specific to certain surface types. When supplying data from input tables to plot layers, the coordinate values you need to supply (and hence the corresponding parameter names) are determined not by the layer type, but by the surface type. For instance, point positions for layer N on a 2-d Cartesian surface (plot2plane command) are given using parameters xN and yN, but when plotting to the celestial sphere (plot2sky command) you supply lonN and latN).

The following list summarises the available surface types and their corresponding positional coordinates.

Plane (plot2plane)

2-dimensional Cartesian axes. Positional coordinates are supplied as x, y pairs. Note that this command can also be used to draw histograms.

Sky (plot2sky)

Celestial sphere. Positional coordinates are supplied as lon, lat pairs, giving longitude and latitude in decimal degrees. A number of different projections are available, and conversion between different celestial coordinate systems can also be performed. You could use it for other spherical coordinate systems too (like the surface of a planet).

Cube (plot2cube)

3-dimensional Cartesian axes. Positional coordinates are supplied as x, y, z triples.

Sphere (plot2sphere)

3-dimensional isotropic space with spherical polar coordinates. Positional coordinates are supplied as lon, lat, r triples, giving longitude and latitude in decimal degrees, and radius in an arbitrary unit. The plotting surface (space) is similar to Cube, except that the unit distance is always the same in all three directions.

Time (plot2time)

2-dimensional axes, but the horizontal axis represents time. The axis may be labelled in various ways (ISO-8601 dates, decimal year, MJD etc). Positional coordinates are supplied as t, y pairs. How to provide a data value representing a time is somewhat under-documented, but reading data from a time-sensitive format such as CDF will give column values that can be used as times. This surface type is somewhat experimental, and the plot2time command currently lacks some important features.