Dear Ester,

Here is that plot

# this is the original code Rasmus E. Benestad 22.08.2010
# 
# Reference for data: Fetterer, F., K. Knowles, W. Meier, and M. Savoie.
# 2002, updated 2009. Sea Ice Index. Boulder, CO: National Snow and Ice
# Data Center. Digital media.  http://nsidc.org/data/g02135.html

ftp1 <- "ftp://sidads.colorado.edu/DATASETS/NOAA/G02135/north/daily/data/NH_seaice_extent_final.csv"
ftp2 <- "ftp://sidads.colorado.edu/DATASETS/NOAA/G02135/north/daily/data/NH_seaice_extent_nrt.csv"
image.name <- "http://www.realclimate.org/images//sea-ice-Hudson-Bay-Dec12-2009.jpg"

col.names <- c("Year", "Month", "Day", "Extent", "Missing", "Source_Data")
nisdc1 <- read.table(ftp1, sep = ",", skip = 2, col.names = col.names)
nisdc2 <- read.table(ftp2, sep = ",", skip = 2, col.names = col.names)
nisdc <- data.frame(Year = c(nisdc1$Year, nisdc2$Year), Month = c(nisdc1$Month, 
    nisdc2$Month), Day = c(nisdc1$Day, nisdc2$Day), Extent = c(nisdc1$Extent, 
    nisdc2$Extent), Missing = c(nisdc1$Missing, nisdc2$Missing))

#  Here is the additional code
require(xtsExtra)

## Loading required package: xtsExtra

## Loading required package: zoo

## Attaching package: 'zoo'

## The following object(s) are masked from 'package:base':
## 
## as.Date, as.Date.numeric

## Loading required package: xts

## Attaching package: 'xtsExtra'

## The following object(s) are masked from 'package:xts':
## 
## plot.xts

nisdc.date = as.Date(with(nisdc, paste(Year, Month, Day)), "%Y %m %d")
# I think the data is updated regularly but I haven't checked how often
max(nisdc.date)

## [1] "2012-09-21"

nisdc.df = data.frame(Date = nisdc.date, Extent = nisdc$Extent, Missing = nisdc$Missing)
row.names(nisdc.df) = nisdc.date
require(ggplot2)

## Loading required package: ggplot2

qplot(x = Date, y = Extent, data = nisdc.df, geom = "line") + theme_bw() + geom_smooth()

## geom_smooth: method="auto" and size of largest group is >=1000, so using
## gam with formula: y ~ s(x, bs = "cs"). Use 'method = x' to change the
## smoothing method.