Almost all minicomputers and every PC uses this convention. This primitive routine is all that is needed for drawing. You must arrange to insert the color change information into the plotfile following the rules of your system. Even if your system does not support color plotting, a dummy version of this routine that simply returns must be provided to satisfy the loader. All the necessary routines for writing letters and numerals are provided in Letter.
Letter includes several fonts, the Greek alphabet and many mathematical symbols.
See the Plotxy documentation for the mechanism to invoke these features.