Plot a flow duration curve

FDC.R

FDC <- function(Q, xnormal=TRUE, xlab='%time flow equalled or exceeded',
                ylab='flow',Qreturn=FALSE, plotFlow=FALSE, add=FALSE, alpha=0.5, ...) {
  
  # Q - flow data (vector of flow values)
  # xnormal - logical value to determine if the percentiles should be plotted in a normal (probit) scale
  # Qreturn - logical value indicating if quantiles should be returned
  # plotFlow - logical value indicating if a time series of flow should be overplotted on flow duration curve
  # add - should fdc be added to an existing plot?
  # alpha - transparency value for overplotting of flow time series
  
  
  
  if(!xnormal){
    y <- quantile(Q, probs=seq(0,1,0.01), na.rm=TRUE, type=8)
    
    
    if(add){
      lines(seq(100,0,-1),y,...)
    } else {
      plot(seq(100,0,-1),y,  type='l',xlab=xlab, ylab=ylab,yaxt='n',...  )
      axis(side=2, at=axTicks(2), labels=prettyNum(axTicks(2), scientific=FALSE))
    }
  } else {
    
    # make labels
    my.labels <- c(1,2,5,seq(10,90,10),95,98,99)
    my.z <- qnorm(my.labels/100)
    
    #quantile calculations
    my.probs <- seq(0,1,0.01)
    y <- quantile(Q, probs=my.probs, na.rm=TRUE, type=8)
    
    # plot
    if(add){
      lines(qnorm(1-my.probs),y, ...) 
    } else {
      plot(qnorm(1-my.probs),y, xaxt='n',yaxt='n',xlab=xlab, ylab=ylab, type='l',...)
      axis(side=1, at=my.z,labels=my.labels)
      axis(side=2, at=axTicks(2), labels=prettyNum(axTicks(2), scientific=FALSE))
    }
    
  }
  if(plotFlow){ # should a time series of data be added to the flow duration curve?
    par(new=TRUE)
    plot(Q, type='l', col=rgb(0,0,0,alpha=alpha), xaxt='n', ann=FALSE, yaxt='n',...)
  }
  
  if(Qreturn) return(y) # return quantiles if requested
  invisible()
  
}