Let’s see together a simple example that explain how to draw a zipf law observed on a long english text.
If you’re not familiar with zipf law, simply put it states that the product of the rank (R) of a word and its frequency (F) is roughly constant. This law is also know under the name “principle of the least effort” because people tends to use the same words often and rarely use new or different words.

Step 1 : Install gnuplot

For mac, check this.
For linux, depending on your distrib it should be as simple as an apt-get install (for ubuntu you can check this howto).
For windows you can either go the “hard” way with cygwin + X11 (see Part 1,4 and 5 of those instructions) or the easy way by clicking on pgnuplot.exe located in the gpXXXwin32.zip located here (this last solution may be also easier if you want to have copy/paste between the gnuplot terminal and other windows).

Step 2: Generate the Zipf Law data using Java and Moby Dick!

As I told you above, gnuplot is particularly simple for drawing a set of generated coordinates. All you have to do is to generated a file containing on each line a couple of coordinates.

For the sake of the example, I will use the full raw text of Moby Dick to generate the points. The goal is to generate a list of points of the form x y where x represents the rank of the word (the more frequent the word is, the higher its rank) and y represents its number of occurrences.

Find below the java code I used to do that. If you want to execute it, you will need lucene and the google collections (soon to become part of guava) libraries.

import java.io.File;
import java.io.FileReader;
import java.io.IOException;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;
 
import org.apache.lucene.analysis.Token;
import org.apache.lucene.analysis.TokenStream;
import org.apache.lucene.analysis.standard.StandardAnalyzer;
 
import com.google.common.collect.HashMultiset;
import com.google.common.collect.Multiset;
import com.google.common.collect.Multiset.Entry;
 
public class ZipfLawOnMobyDick {
	public static void main(String[] args) throws IOException {
 
		//Multiset for storing word occurrences
		Multiset multiset = HashMultiset.create();
 
		//Creating a standard analyzer with no stop words (we need them to observe the zipf law)
		String[] STOP_WORDS = {};
		StandardAnalyzer analyzer = new StandardAnalyzer(STOP_WORDS);
 
		//Initializing the multiset by parsing the whole content of Moby Dick
		TokenStream stream = analyzer.tokenStream("content", new FileReader(new File("C:\\moby_dick.txt")));
		Token token = new Token();
		while ((token = stream.next(token)) != null){
			multiset.add(token.term());
		}
 
		//Sorting the multiset by number of occurrences using a comparator on the Entries of the multiset
		List> l = new ArrayList>(multiset.entrySet());
		Comparator> occurence_comparator = new Comparator>() {
			public int compare(Multiset.Entry e1, Multiset.Entry e2) {
				return e2.getCount() - e1.getCount() ;
			}
		};
		Collections.sort(l,occurence_comparator);
 
		int rank = 1;
		for( Multiset.Entry e : l ){
			System.out.println(rank+"\t"+e.getCount());
			rank++;
		}
	}
}

This will generate the following output (the set of coordinates) that you can put in a file called moby_dick.gp. If you’re curious about what are the 100 hottest keywords of the whole text you can check them here.

Step 3: Drawing using gnuplot

What you can do first is simply to type the following command in the gnuplot console (you have to be on the same directory as the moby_dick.gp file):

plot [0:500][0:16000] "moby_dick.gp"

It simply draws the points and rescale the range of x and y respectively to [0:500] and [0:16000] so we can see something.
Play with the ranges to see the differences.
If you want the dots to be connected, just type:

plot [0:500][0:16000] "moby_dick.gp" with lines

If you want to add some legends, you can put some labels and arrows.
Here is an example of a gnuplot script that will set some information on the graph (you can simply copy/paste it in the gnuplot console):

set xlabel "word rank"
set ylabel "# of occurrences"
set label 1 "the word ranked #14 occurs 1753 times" at 70,4000
set arrow 1 from 65,3750 to 15,1800
plot [0:500][0:16000] "moby_dick.gp

As you can see it is pretty straightforward. You can play with the coordinates to adjust where to put the labels and arrow.
You will obtain this graph (click to enlarge):

To export it as a png file just type:

set terminal png
set output "moby_dick.png"
plot [0:500][0:16000] "moby_dick.gp"

You also might want to try a log scale on the vertical axis as to not waste the majority of the graph’s scale (thanks Bob for the remark).
To do so, you can simply type in the gnuplot console:

set logscale y

by plotting within the range [1:3000][5:10000], you’ll obtain:

Finally, you might want to use a log-log scale that are traditionally used to observe such power laws. Just set the logscale for x as you did for y and you’ll obtain:

You can of course add as much eye candies as you want (the demo page of the gnuplot website gives tons of example).

Also, there are probably dozens of ways to draw the same thing, I just loved the fun and simplicity of that one.