The plotutils Package

The GNU plotutils package contains software for both programmers and technical users. Its centerpiece is libplot, a powerful C/C++ function library for exporting 2-D vector graphics in many file formats, both vector and bitmap. On the X Window System, it can also do 2-D vector graphics animations.

libplot is device-independent, in the sense that its API (application programming interface) does not depend on the type of graphics file to be exported. A Postscript-like API is used both for file export and for graphics animations. A libplot programmer needs to learn only one API: not the details of many graphics file formats.

The package also contains command-line programs for plotting scientific data, such as GNU graph for XY plotting. Many of them use libplot to export graphics. So, they can export graphics in any of libplot's supported formats, such as

• SVG (Scalable Vector Graphics format), the XML-based format for Web graphics, which is standardized by the W3 Consortium.
• PNG, the open format for bitmap graphics. (Whenever a bitmap output format is being generated, libplot converts Postscript-style paths to bitmaps.)
• The format used by the free xfig drawing editor. (Edit a 2-D drawing with a mouse, after generating it programmatically!)
• Postscript itself.

The current version of the plotutils package is 2.6, released September 2009. It can be installed on GNU/Linux, FreeBSD, and Unix systems.

The plotutils package comes with a 185-page manual. Here is an English-language version of the manual, and here is a Japanese-language version of the manual. People working through the many examples given in the manual should look at the expected output (PDF). (Thanks to Daisuke TOMINAGA for the last two items!).

The package is free software. Its source code is distributed as a 3.7 megabyte gzipped tar file. Here is how you can get it.

What command-line programs does the package contain?

It includes

• GNU graph, which plots 2-D datasets or data streams in real time. Being designed for command-line use, it can be used in shell scripts. It produces output on an X Window System display, in SVG format, in PNG format, in PNM format, in pseudo-GIF format, in WebCGM format, in Illustrator format, in Postscript format, in PCL 5 format, in HP-GL/2 format, in Fig format (editable with the xfig drawing editor), in ReGIS format, in Tektronix format, or in GNU Metafile format. Output in Postscript format may be edited with the idraw drawing editor. idraw is available in the ivtools package from Vectaport, Inc. Both xfig and idraw are free software.
• GNU plot, which translates GNU Metafile format to any of the other formats.
• GNU tek2plot, for translating legacy Tektronix data to any of the above formats.
• GNU pic2plot, for translating the pic language (a scripting language for designing box-and-arrow diagrams) to any of the above formats. The pic language was designed at Bell Labs as an enhancement to the troff text formatter.
• GNU plotfont, for displaying character maps of the fonts that are available in the above formats.
• GNU spline, which does spline interpolation of data. It normally uses either cubic spline interpolation or exponential splines in tension, but it can function as a real-time filter under some circumstances.
• GNU ode, which numerically integrates a system consisting of one or more ordinary differential equations.

We developed these command-line programs to replace the Unix command-line programs graph, plot, and spline. The GNU versions are far more powerful, and are free software.

What graphics library does the package contain?

GNU libplot, a C/C++ function library for device-independent 2-D vector graphics. GNU libplot is compatible with the traditional Unix libplot library, but is far more powerful. It is installed as part of the package. On systems that support shared libraries, it is installed as a shared library. A C++ class library called libplotter, which provides an object-oriented interface to libplot's functionality, is optionally installed as well.

GNU libplot and libplotter support all the output formats mentioned above (X11, SVG, PNG, PNM format, pseudo-GIF, WebCGM, Illustrator format, idraw-editable Postscript, PCL 5, Fig format, HP-GL/2, ReGIS, Tektronix, and GNU Metafile). They can produce animated pseudo-GIFs, and smooth, double-buffered animations on any X Window System display. The libplot imaging model is similar to Postscript's. In any output format, they can draw the following.

• Graphical objects such as lines, circles and ellipses, points, and marker symbols; also paths. A path is defined as in Postscript as a sequence of line segments, arcs (either circular or elliptic), or Bezier curve segments (either quadratic or cubic). Paths may be open or closed, and they may be dashed. The filling of paths is supported (fill color, as well as pen color, may be set arbitrarily).
• Text strings, in many fonts. Text strings may include subscripts and superscripts, and may include characters chosen from more than one font in a typeface. The X11, SVG, WebCGM, Illustrator, Postscript, and xfig drivers support the 35 standard Postscript fonts, and the SVG, Illustrator, PCL 5 and HP-GL/2 drivers support the 45 standard PCL 5 fonts. All drivers support a set of 22 Hershey vector fonts. This includes HersheyCyrillic, a Russian font that uses the KOI8-R encoding, and HersheyEUC, a Japanese font that uses the 8-bit EUC-JP encoding. Japanese text strings may include both syllabic characters (Hiragana and Katakana) and ideographic characters (Kanji). A library of over 600 Kanji is built in.

When using libplot or libplotter, a programmer draws vector graphics in a `user frame', rather than in the device frame. As in Postscript, the user frame may be transformed into the device frame by an arbitrary affine map. Scaling, rotation, shearing, and translation are all supported.

Does the package really produce GIF files?

Even though the plotutils package can produce what appear to be GIFs and animated GIFs, it does not transgress any patents covering the LZW compression algorithm. The reason is that instead of LZW encoding, it uses run-length encoding, which is not patentable. To avoid confusion, we call the files produced by the package `pseudo-GIF files'.

What related software is available elsewhere?

A simple piechart plotting program that illustrates the use of GNU libplot is available here. It was contributed by Bernhard Reiter. Chris Elliott has developed a libplot-based program called ascii_chart, which takes data in a two-column format and prepares a piechart plot or a line plot.

There is a Python wrapper for libplot, and a SWIG wrapper too. libplot drawing functions can also be used from a Perl module (Graphics::Libplot).

Who developed the plotutils package?

The two primary authors of the plotutils package are Robert Maier and Nick Tufillaro (who developed ode). Many other people have also contributed. The rasterization code used by the export filters, which is distributed as a separate package, is based on the scan-conversion code in the sample X Window System server.

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