俺是R语言白丁。请帮忙解释下面一段R程序，比如：哪些变量，关系，每个命令和参数的含义，语句的含义和结果等等。越详细越好。。对解释最佳者，给予300论坛币略表心意。
另外，附上相关数据供参考。。
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gdp <- read.csv("GDP P chain volume flat.csv",
na.strings="..", check.names=TRUE)
# Create shades of colors between blue and red for labels
shades <- colorRampPalette(c("blue", "red"))(nrow(gdp))
# Exploration of wholesale and retail trade - extract trend series
wholesale.d <- decompose(ts(gdp$Wholesale.Trade, frequency=4, start=c(1987,2)),
type="multiplicative")
wholesale <- wholesale.d$trend
retail.d <- decompose(ts(gdp$Retail.Trade, frequency=4, start=c(1987,2)),
type="multiplicative")
retail <- retail.d$trend
# Exploratory plot 1
plot(wholesale,retail, bty="l")
# Exploratory plot 2
plot(cbind(wholesale,retail), bty="l")
# Main plot
library(MASS)
eqscplot(diff(wholesale), diff(retail), bty="l", cex=.5, col=c("grey60", shades),
xlab="Growth since last quarter in wholesale contribution to GDP, $m",
ylab="Growth since last quarter in retail contribution to GDP, $m")
grid(col="grey90", lty=1)
# Draw diagonal line showing when the two variables are equal
abline(0,1)
# Draw line showing regression
abline(lm(diff(retail) ~ diff(wholesale), na.action="na.omit"), lty=2)