updates doc

#' @param labels a character vector giving labels (must be same length as breaks).

#' @param limits a numeric vector of length two providing limits of the scale.

#' @param scale NULL or a \code{Scale} object.

#' @param ... other parameters passing to \code{\link[ggplot2]{scale_fill_gradientn}}.

#' @param ... other parameters passing to \code{scale_fill_gradientn}.

#' @rdname set_style

#' @author Hou Yun

#' @export

\arguments{

\item{x}{a hyplot object.}

\item{...}{other parameters passing to \code{\link[ggplot2]{scale_fill_gradientn}}.}

\item{...}{other parameters passing to \code{scale_fill_gradientn}.}

\item{colours}{vector of colours to use for n-colour gradient.}