Visualized : plot

The black dots represent the monthly sales figures.
The blue line is the line fitted by R's lm() function through the sales dots.
The red dot is the predicted sale for month 13.
In each case the red dot lies on the blue line, which proves the correctness.

The R code


    df=read.table("predicion.csv",sep=",",na.strings="null")

    df=df[!is.na(df$V3),]
    names(df)= c("shop","product","unit","month")


    dev.off()
    shops=unique(df$shop)
    products=unique(df$product)
    dev.new(width=8, height=16)
    par( mfcol=c(6,2) )
    for ( sh in shops) {
        for ( pr in products) { 
            y=df[ df$shop==sh & df$product==pr,"unit"]
            x=df[ df$shop==sh & df$product==pr,"month"]

            plot(x,y,pch=19,main=paste(sh,pr))
            fit=lm( y~x) 
            abline(fit,col="blue",lwd=3)

            xx=13
            yy=y[x==xx]
            points( xx,yy,pch=19, col="red",lwd=5)
    }}

Notes by Data Munging Ninja. Generated on nini:/home/willem/sync/20151223_datamungingninja/simplesalesprediction at 2016-06-25 10:02