ggvis2: Interactive grammar of graphics

#' @section Properties:
#'
#' You can set the following mark properties:
#'
#' \itemize{
#' \item  x  The first (typically left-most) x-coordinate.
#' \item  x2 The second (typically right-most) x-coordinate.
#' \item  width The width of the mark (if supported).
#' \item  y The first (typically top-most) y-coordinate.
#' \item  y2 The second (typically bottom-most) y-coordinate.
#' \item  height The height of the mark (if supported).
#' \item  opacity The overall opacity.
#' \item  fill The fill color.
#' \item  fillOpacity The fill opacity
#' \item  stroke The stroke color.
#' \item  strokeWidth The stroke width, in pixels.
#' \item  strokeOpacity The stroke opacity.
#' \item  size [symbol] The pixel area of the symbol. For example in the case
#'   of circles, the radius is determined in part by the square root of the size
#'   value.
#' \item  shape [symbol] The symbol shape to use. One of circle (default),
#'   square, cross, diamond, triangle-up, or triangle-down (symbol only)
#' \item  innerRadius [arc] The inner radius of the arc, in pixels.
#' \item  outerRadius [arc] The outer radius of the arc, in pixels.
#' \item  startAngle [arc] The start angle of the arc, in radians.
#' \item  endAngle [arc] The end angle of the arc, in radians.
#' \item  interpolate [area, line] The line interpolation method to use. One
#'   of linear, step-before, step-after, basis, basis-open, cardinal,
#'   cardinal-open, monotone.
#' \item  tension [area, line] Depending on the interpolation type, sets the
#'   tension parameter.
#' \item  url [image] The URL from which to retrieve the image.
#' \item  align [image, text] The horizontal alignment of the object. One of
#'   left, right, center.
#' \item  baseline [image, text] The vertical alignment of the object. One of
#'   top, middle, bottom.
#' \item  text [text] The text to display.
#' \item  dx [text] The horizontal margin, in pixels, between the text label
#'   and its anchor point. The value is ignored if the align property is center.
#' \item  dy [text] The vertical margin, in pixels, between the text label
#'   and its anchor point. The value is ignored if the baseline property is
#'   middle.
#' \item  angle [text] The rotation angle of the text, in degrees.
#' \item  font [text] The typeface to set the text in (e.g., Helvetica Neue).
#' \item  fontSize [text] The font size, in pixels.
#' \item  fontWeight [text] The font weight (e.g., bold).
#' \item  fontStyle [text] The font style (e.g., italic).
#' }
#'
#' To each property, you can assign any property object (\code{\link{prop}})
#' either locally (i.e. in the mark), or in a parent \code{layer}.