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\details{
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\details{
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  Unit objects allow the user to specify locations and dimensions
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  Unit objects allow the user to specify locations and dimensions
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  in a large number of different coordinate systems.  All drawing
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  in a large number of different coordinate systems.  All drawing
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  occurs relative to a viewport and the \code{units} specifies
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  occurs relative to a viewport and the \code{units} specifies
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  what coordinate system to use within that viewport.
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  what coordinate system to use within that viewport.
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  Possible \code{units} (coordinate systems) are:
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  Possible \code{units} (coordinate systems) are:
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  \describe{
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  \describe{
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    \item{\code{"npc"}}{Normalised Parent Coordinates (the default).
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    \item{\code{"npc"}}{Normalised Parent Coordinates (the default).
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      The origin of the viewport is (0, 0) and the viewport has a width
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      The origin of the viewport is (0, 0) and the viewport has a width
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      and height of 1 unit.  For example, (0.5, 0.5) is the centre of
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      and height of 1 unit.  For example, (0.5, 0.5) is the centre of
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      the viewport's \code{xscale} and \code{yscale}.}
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      the viewport's \code{xscale} and \code{yscale}.}
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    \item{\code{"snpc"}}{Square Normalised Parent Coordinates.
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    \item{\code{"snpc"}}{Square Normalised Parent Coordinates.
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      Same as Normalised Parent Coordinates, except gives the same answer for
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      Same as Normalised Parent Coordinates, except gives the same answer for
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      horizontal and vertical locations/dimensions.  It uses the
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      horizontal and vertical locations/dimensions.  It uses the
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      \emph{lesser} of npc-width and npc-height.  This is useful for making
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      \emph{lesser} of npc-width and npc-height.  This is useful for making
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      things which are a proportion of the viewport, but have to be square
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      things which are a proportion of the viewport, but have to be square
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      (or have a fixed aspect ratio).}
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      (or have a fixed aspect ratio).}
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    \item{\code{"strwidth"}}{Multiples of the width of the string
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    \item{\code{"strwidth"}}{Multiples of the width of the string
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      specified in the \code{data} argument.  The font size is
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      specified in the \code{data} argument.  The font size is
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      determined by the pointsize of the viewport.}
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      determined by the pointsize of the viewport.}
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    \item{\code{"strheight"}}{Multiples of the height of the string
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    \item{\code{"strheight"}}{Multiples of the height of the string
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  A number of variations are also allowed for the most common units.
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  A number of variations are also allowed for the most common units.
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  For example, it is possible to use \code{"in"} or \code{"inch"}
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  For example, it is possible to use \code{"in"} or \code{"inch"}
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  instead of \code{"inches"} and \code{"centimetre"} or
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  instead of \code{"inches"} and \code{"centimetre"} or
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  \code{"centimeter"} instead of \code{"cm"}.
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  \code{"centimeter"} instead of \code{"cm"}.
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  A special \code{units} value of \code{"null"} is also allowed,
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  A special \code{units} value of \code{"null"} is also allowed,
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  but only makes sense when used in specifying widths of columns
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  but only makes sense when used in specifying widths of columns
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  or heights of rows in grid layouts (see \code{\link{grid.layout}}).
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  or heights of rows in grid layouts (see \code{\link{grid.layout}}).
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  The \code{data} argument must be a list when the \code{unit.length()}
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  The \code{data} argument must be a list when the \code{unit.length()}
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  is greater than 1.  For example,
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  is greater than 1.  For example,
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\preformatted{  unit(rep(1, 3), c("npc", "strwidth", "inches"),
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\preformatted{  unit(rep(1, 3), c("npc", "strwidth", "inches"),
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  data = list(NULL, "my string", NULL))
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  data = list(NULL, "my string", NULL))
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}.
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}.
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  It is possible to subset unit objects in the normal way
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  It is possible to subset unit objects in the normal way
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  (e.g., \code{unit(1:5, "npc")[2:4]}), but a special function
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  (e.g., \code{unit(1:5, "npc")[2:4]}), but a special function
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  \code{unit.c} is provided for combining unit objects.
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  \code{unit.c} is provided for combining unit objects.
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  Certain arithmetic and summary operations are defined for
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  Certain arithmetic and summary operations are defined for
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  work exactly the same as
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  work exactly the same as
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  \code{"lines"}, \code{"char"}, \code{"strwidth"},
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  \code{"lines"}, \code{"char"}, \code{"strwidth"},
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  \code{"strheight"}.
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  \code{"strheight"}.
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}
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}
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\value{
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\value{
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  An object of class \code{"unit"}.
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  An object of class \code{"unit"}.
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}
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}
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\author{Paul Murrell}
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\author{Paul Murrell}
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\seealso{
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\seealso{
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  \code{\link{unit.c}}
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  \code{\link{unit.c}}
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}
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}