So I’m getting my daily fix of r-bloggers.com and I encounter this post. Hey, I figure, if polish can draw his country flag using R, why can’t lithuanian do it? 🙂 It should be easy, as cut and paste, right? It turned out not so easy in the end, but not that hard either. Here is the final result. The R code with the description of the process is below.

The hardest part was to get the map of the Lithuania. It turns out that the maps package used to obtain the map of Poland has out-dated maps, circa 1980. There was no Lithuania at that time, curse you Soviets 🙂

After searching a bit, I did not find the R package with the updated map, but I found out how to get more recent map into R. First you need to download the following file from this site. The simplified map from this site is in R package maptools, but it seems the full version is not available as an R package.
You have to unzip the file and then you import it in the following way: 

library(maptools)
world<-readShapeSpatial("~/Downloads/TM_WORLD_BORDERS-0.3/TM_WORLD_BORDERS-0.3.shp")

lithuania<-world[world$ISO2=="LT",]
@

To get into more readable format the following lines are necessary. I found them on github ggplot2 page. Thanks to Haddley Wickham: 

library(gpclib)
gpclibPermit()
library(ggplot2)
lt<-fortify(lithuania,region="ISO2")

> head(lt)
      long      lat order  hole piece group id
1 25.00000 56.29555     1 FALSE     1  LT.1 LT
2 25.07250 56.21915     2 FALSE     1  LT.1 LT
3 25.08111 56.21055     3 FALSE     1  LT.1 LT
4 25.09694 56.20110     4 FALSE     1  LT.1 LT
5 25.10611 56.19721     5 FALSE     1  LT.1 LT
6 25.13388 56.18888     6 FALSE     1  LT.1 LT

As you see in the end we get the data.frame with longitude and lattitudes and additional info. The relevant additional info in our case is in pieces column. Lithuania consists of 2 separate pieces: the mainland and Curonian spit. Since only part of Curonian spit belongs to Lithuania, it is not connected to Lithuanian mainland. So in the map it looks like an island, when in fact it is not.

The points in the data.frame are ordered, so you can plot them immediately: 

plot(lt1,axes=FALSE,xlab="",ylab="",type="l")
polygon(lt2)

Now we have the shape, we need only to fill in the colors. To do that we need to divide the 2 polygons into 3 pieces with equal heights.

It is easy to cut the polygon into 2 pieces along the horizontal line. Here are the functions I used to do that: 

x.mid <- function(x1, x2, y.mid) {
      c(x1[1] + ((x2[1] - x1[1]) / (x2[2] - x1[2])) * (y.mid - x1[2]),y.mid)
}

cut.poly.in.half <- function(xy,cut.y.point) {
    cond <- xy[,2]>cut.y.point
    if(!is.matrix(xy))xy <- as.matrix(xy)
    if(sum(cond)==0 | sum(!cond)==0) {
        warning("Whole polygon is either below orabove the cut-line, original polygon is returned")
        return(xy)
    }
    
    dcond <- diff(cond)
    if(sum(dcond)==0) {
        
        tmp<- sort(c(which.min(diff(cond)),which.max(diff(cond))))
        start.y <- tmp[1]
        end.y <- tmp[2]


        start <- x.mid(xy[start.y,],xy[start.y+1,],cut.y.point)    
        
        end <- x.mid(xy[end.y,],xy[end.y+1,],cut.y.point)
  
        top <- rbind(xy[1:start.y,],start,end,xy[(end.y+1):nrow(xy),])
        bottom <- rbind(start,xy[(start.y+1):end.y,],end)
        if(top[1,2]<cut.y.point) {
            tmp <- top
            top <- bottom
            bottom <- tmp
        }
        list(top=top,bottom=bottom)
    }
    
}

I borrowed some code from Bogumił Kamiński. The function just adds two points to the array of points. It checks whether the polygon is cuttable, i.e. the horizontal line goes through the middle. It also assumes, that the line cuts the the polygon into 3 pieces, if we imagine the polygon as the line with the start and an end, which basically what polygon is in R. It is easy to add the code for the case of 2 pieces, but this case is rare, and I was lazy, so there you go:).

Now let us start the cutting: 

##We need to cut the polygon of all Lithuania into three equal parts
##First cut out the top part
aa <- cut.poly.in.half(as.matrix(lt1),cut.points[2])
##Now divide the remaining part into halves
bb <- cut.poly.in.half(aa$bottom,cut.points[1])
##Curonian spit part only needs to be cut into two parts
cc <- cut.poly.in.half(lt2,cut.points[2])
@

Now get the colors of Lithuanian flag, and do the final step: 

##Colors of Lithuanian flag, from top to bottom.
##Gzr stands for geltona, zalia, raudona: yellow, green, red in lithuanian.
gzr <- c("#fdb913","#006a44","#c1272d")
##Colour the top 
polygon(aa$top,col=gzr[1])
polygon(cc$top,col=gzr[1])
##Colour the middle
polygon(bb$top,col=gzr[2])
polygon(cc$bottom,col=gzr[2])
##Colour the bottom
polygon(bb$bottom,col=gzr[3])

Using this code it is more or less easy to replicate this exercise for other countries which have flags with where colours are horizontal and there are no additional structures. Flags with vertical colours should be easy too. But flags with more structure as flag of Finland, or United Kingdom would be naturally harder.