Categorizing my expenses

In order to analyse my expenses, a classification scheme is necessary. I need to identify categories that are meaningful to me. I decided to go with the “Classification of Individual Consumption by Purpose” (COICOP), for three reasons:

• It is made by people who have thought more about consumption classification than I ever will.
• It is feasible to assign bank transactions and tracked cash spendings to one of the 12 top level categories.
• It is widely used by statistics divisions, e.g. the Federal Statistical Office of Germany, Eurostat, and the UN. This means I can do social comparisons: In which categories do I spend more money than the average? Do the prices I pay rise faster than the price indices suggest?

So I classified my last year’s expense data according to COICOP. Here is a chart showing the portions of the categories for each month:

For me, the holidays, prepared in August and traveled in September (shown as unknown expenses), are much more dominant than I expected. Except for the new glasses in September I did not make any larger investments.

I like this kind of chart more than stacked bar charts because the history for each category is very visible. This chart is called inkblot chart. I stumbled on it on junk charts, asked how to implement it in R on StackOverflow, and included a revised version in the latest pft package. See below for more information.

You find the inkblot function in the pft package. It is work in progress. The current version is experimental and I did not even try to submit it to CRAN. You can inspect it by downloading it from my personal repository:

> install.packages(
+   "pft", 
+   repos=c(getOption("repos"),
+      "http://userpage.fu-berlin.de/~kweinert/R"),
+   dependencies=c("Depends", "Suggests")
+ )

If you are not a Windows user, you may have to add the type="source" argument. Or you can copy the function from below:

> inkblot
function(series, names.arg= NULL, col="grey", border=par("fg"), main=NULL,
na.replace=FALSE, min.height=NULL, 
major.ticks = "auto", major.format="",
grid=TRUE, lty.grid="dashed", col.grid="lightgrey", 
verbose=getOption("verbose"), ...) {  
if(!xtsible(series)) stop("invalid 'series' argument, xts compatible expected.")
if(!na.replace) 
series <- na.fail(series)
else 
series[which(is.na(series))] <- 0

if(any(series < 0)) stop("invalid 'series' argument, non-negative values expected.")
if(is.null(names.arg)) names.arg <- colnames(series)
if(major.format=="") major.format="%Y"  # hack for roxygen
if(length(col)!=dim(series)[2]) col <- rep(col, length.out=dim(series)[2])
old.par <- par(no.readonly = TRUE);on.exit(par(old.par))
par(...)

if(is.null(min.height)) {
ytotal <- 0  
for(i in 1:dim(series)[2]) ytotal <- ytotal + max(series[, i])
min.height <- ytotal * par("cex") / (1.5 * dim(series)[2])
}  
if(verbose) cat("min.height is ", min.height, "\n")

ytotal <- 0  
for(i in 1:dim(series)[2]) {  
ytotal <- ytotal + max(series[, i], min.height)  
}  
if(verbose) cat("ytotal is ", ytotal, "\n")

x <- xy.coords(.index(series), series[, 1])$x
plot(x, 1:length(x), type="n", ylim=c(0,1)*ytotal, yaxt="n", xaxt="n", bty="n", ylab="", xlab="", main=main) 
ep <- axTicksByTime(series, ticks.on=major.ticks, k=1, labels=TRUE, format.labels=major.format)
axis(side=1, at = x[ep], labels = names(ep), las=1, tick=FALSE)
if (grid) abline(v=x[ep], col=col.grid, lty=lty.grid)

catNumber <- 1  
offset <- 0  
res <- NULL
for(catNumber in 1:dim(series)[2]) {  
y <- 0.5 * as.vector(series[,catNumber])  
offset <- offset + max(max(y), 0.5*min.height)
polygon(c(x, rev(x)), c(offset+y, offset-rev(y)), col=col[catNumber], border=border)
if(!is.null(names.arg)) mtext(text=names.arg[catNumber], side=2, line=0, at=offset, las=2, adj=1, cex=par("cex")*par("cex.axis"))  
res <- c(res, offset)
offset <- offset + max(max(abs(y[!is.na(y)])), 0.5*min.height)  
catNumber <- catNumber + 1   
}  
return(res)
}

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