There are two Hershey font character sets included with PLplot. These are known as the standard and extended character sets. The standard character set is a subset of the extended set. It contains 177 characters including the ascii characters in a normal style font, the Greek alphabet and several plotter symbols. The extended character set contains almost 1000 characters, including four font styles, and several math, musical and plotter symbols.

The extended character set is loaded into memory automatically when PLplot is initialized. The standard character set is loaded by calling plfontld. The extended character set requires about 50 KBytes of memory, versus about 5 KBytes for the standard set. plfontld can be used to switch between the extended and standard sets (one set is unloaded before the next is loaded). plfontld can be called before plstar.

When the extended character set is loaded there are four different font styles to choose from. In this case, the routine plfont sets up the default Hershey font for all character strings. It may be overridden for any portion of a string by using an escape sequence within the text, as described in the section called “Escape sequences in text”. For the Hershey font system (but not the unicode font system, see below) this routine has no practical effect when the standard font set is loaded. The default font (1) is simple and fastest to draw; the others are useful for presentation plots on a high-resolution device.

The font codes are interpreted as follows:

The advantages of the unicode fonts over the more traditional PLplot Hershey fonts are the availability of many additional glyphs (including mathematical symbols and glyphs from other than western-European languages); support of complex text layout languages for a substantial subset (see the section called “The svg device driver” and the section called “The psttf, cairo, qt, and wxwidgets device drivers”) of the devices that support the Unicode font system; and much better display of characters on computer screens using anti-aliasing and hinting.

The unicode font system can use font specification methods that were designed for the Hershey fonts to specify the unicode font. For this case plfont internally calls plsfci using four different FCI values to choose unicode font attributes similar to the 4 kinds of Hershey fonts, and the corresponding Hershey text escape-sequences (see the section called “Escape sequences in text”) designed to override the Hershey font selection are treated similarly. However, for the unicode font system the preferred and much more flexible methods of specifying the unicode font are calling the plsfci routine, calling the plsfont routine which provides a user-friendly interface to plsfci, or using unicode text escape-sequences to override unicode font attributes in the middle of strings (see the section called “FCI”).

We specify the properties of unicode fonts with the FCI (font characterization integer). The FCI is a 32-bit unsigned integer whose most significant hexadecimal digit is set (by ORing 0x80000000 with the FCI value) to distinguish it from a unicode (UCS4) integer (whose maximum value is 0x7fffffff). Users obtain the current FCI by calling plgfci and store a new FCI to be used at the start of each subsequent string using plsfci. The FCI contains three independent hexadecimal values corresponding to font family, font-style, and font weight. These three values can also be obtained and set in a user-friendly way using plgfont and plsfont. These values are also characterized by "hexdigit" (defined as the actual hexadecimal value used for one of the hexadecimal digits) and "hexpower" (defined as the power of 16 or number of hexadecimal places to the left of the "decimal place" in the FCI where the hexdigit is stored). The interpretation of the hexdigit and hexpower values in the FCI are given in Table 3.3, “FCI interpretation”.

Note the maximum value of hexdigit is 7 and the maximum value of hexpower is 6 so there is substantial room for expansion of this scheme. On the other hand, since each font attribute is independent of the rest, what is implemented now gives us a maximum of 30 different font possibilities which is probably more than enough for most plotting purposes.

The routines which draw text all allow you to include escape sequences in the text to be plotted. These are character sequences that are interpreted as instructions to change fonts, draw superscripts and subscripts, draw non-ASCII (e.g. Greek), and so on. All escape sequences start with a number symbol (#) by default. Some language interfaces have the capability of changing this default, but we will assume (#) in the remaining documentation of the escape sequences.

The following escape sequences are defined:

Sections of text can have an underline or overline appended. For example, the string S̅(f̲r̲e̲q̲) is obtained by specifying "#+S#+(#-freq#-)".

Greek letters are obtained by #g followed by a Roman letter. Table 3.4, “Roman Characters Corresponding to Greek Characters” shows how these letters map into Greek characters.

The escape sequences #fn, #fr, #fi, #fs, and #(nnn) are designed for the four Hershey fonts, but an effort has been made to allow some limited forward compatibility so these escape sequences have a reasonable result when unicode fonts are being used. However, for maximum flexibility when using unicode fonts, these 5 Hershey escape sequences should be replaced by using the 4 unicode escape sequences #<0x8nnnnnnn>, #<0xmn>, #<FCI COMMAND STRING/>, and #[nnn] as appropriate.

It should be emphasized that the unicode escape sequences above only work properly for modern unicode-aware devices such as the svg and wxwidgets devices or the very large set of cairo and qt devices. And for those devices the alternative of directly specifying the unicode symbols using UTF-8 encoding of PLplot input strings is much more convenient for users than using the above #[nnn] type of escape sequence. For example, we use UTF-8 strings rather than escape sequences in our standard example 24 to render the word "Peace" in several different languages.

For unicode-aware devices it is possible as well to specify mathematical glyphs (such as ∂, ∇, ∑, ∫, and ∰) using UTF-8 encoding of PLplot input strings. A typical input method in this case is simply to cut and paste the desired mathematical glyph from, e.g., gucharmap to source code being edited by a unicode-aware editor such as emacs. Such input methods may be conveniently used, for example, to specify the very wide range of mathematical symbols that are typically desired for scientific plots.

The routine plschr is used to set up the size of subsequent characters drawn. The actual height of a character is the product of the default character size and a scaling factor. If no call is made to plschr, the default character size is set up depending on the number of subpages defined in the call to plstar or plstart, and the scale is set to 1.0. Under normal circumstances, it is recommended that the user does not alter the default height, but simply use the scale parameter. This can be done by calling plschr with def = 0.0 and scale set to the desired multiple of the default height. If the default height is to be changed, def is set to the new default height in millimeters, and the new character height is again set to def multiplied by scale.

The routine plssym sets up the size of all subsequent characters drawn by calls to plpoin and plsym. It operates analogously to plschr as described above.