By default, libplot's CGM driver emits version 3 CGM format, conforming to
the WebCGM profile.  The output may include the following CGM primitives.
For their semantics, see "The CGM Handbook" (cited below).  Many of the
names should be self-explanatory.

By setting the libplot parameter or environment variable CGM_MAX_VERSION to
"2" or "1", you can produce a CGM file whose version is less than 3.  This
is often desirable, because many CGM viewers can handle only version 1 CGM
files.  If the CGM version is 2 or 1, the primitives below that are not
allowed to appear in version 2 or 1 CGM files will be emulated using
allowed primitives, or simply dropped (if appropriate).  The table
indicates which primitives may appear only in version 2 and higher, 
and 3 and higher.

CGM Delimiter Elements:
	 BEGMF
	 BEGPIC
	 BEGPICBODY
	 ENDPIC
	 ENDMF

CGM Metafile Descriptor Elements:
	 MFVERSION
	 MFELEMLIST
	 MFDESC
	 VDCTYPE
	 INTEGERPREC
	 REALPREC
	 COLRPREC
	 COLRVALUEEXT
	 FONTLIST
	 CHARSETLIST
	 CHARCODING
	 FONTPROP (version 3 or higher)

CGM Picture Descriptor Elements:
	SCALEMODE
	COLRMODE
	LINEWIDTHMODE
	EDGEWIDTHMODE
	MARKERSIZEMODE
	VDCEXT
	BACKCOLR
	LINEEDGETYPEDEF (version 3 or higher)

CGM Control Elements:
	VDCINTEGERPREC
	MITRELIMIT (version 3 or higher)

CGM Graphical Primitive Elements:
	LINE
	MARKER
	RESTRTEXT
	POLYGON
	RECT
	CIRCLE
	ARCCTR
	ELLIPSE
	ELLIPARC
	ARCCTRREV (version 2 or higher)
	POLYBEZIER (version 3 or higher)

CGM Attribute Elements:
	LINETYPE
	LINEWIDTH
	LINECOLR
	MARKERTYPE
	MARKERSIZE
	MARKERCOLR
	TEXTFONTINDEX
	TEXTPREC
	TEXTCOLR
	CHARHEIGHT
	CHARORI
	TEXTALIGN
	CHARSETINDEX
	ALTCHARSETINDEX
	INTSTYLE
	FILLCOLR
	EDGETYPE
	EDGEWIDTH
	EDGECOLR
	EDGEVIS
	LINECAP (version 3 or higher)
	LINEJOIN (version 3 or higher)
	LINETYPEINITOFFSET (version 3 or higher)
	EDGECAP (version 3 or higher)
	EDGEJOIN (version 3 or higher)
	EDGETYPEINITOFFSET (version 3 or higher)
	RESTRTEXTTYPE (version 3 or higher)

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References used when writing the CGM driver include:

    The WebCGM profile, available at http://www.w3.org/TR/REC-WebCGM as an
    official recommendation of the World Wide Web Consortium.

    "The CGM Handbook", L. R. Henderson and A. Mumford (Academic, 1992).

    "CGM and CGI", D. B. Arnold and P. R. Bono (Springer-Verlag, 1988).

    Sourcecode for cd-1.3, also called CD, cgmdraw 1.3, and CGM Draw 1.3, a
    freely available C function library for generating version-1 CGM files.
    cd-1.3 was developed at NIST (the U.S. National Institute for Standards
    and Technology) by G. Edward Johnson (formerly, lorax@nist.gov).
    However, it no longer seems to be available through NIST Web pages.

    Sourcecode for RALCGM, a free viewer for version-1 CGM files developed
    at Rutherford Appleton Laboratory in the U.K.  Available at
    ftp://ftp.cc.rl.ac.uk/pub/graphics/ralcgm/unix/ .

    http://www.posc.org/Epicentre.2_2/GraphicsMetafile/SeismicExtensions/ .
    Contains a GIF image of the section from the CGM specification dealing
    with the format of SDR's (structured data records, used in specifying
    font properties in version-3 CGM files).

    http://www.nist.gov/itl/div897/ctg/cgm_form.htm .
    Sample CGM files, forming test suites, distributed by the US NIST
    (National Institute of Standards and Technology).
