Blogging and Babel

In my last post, I included a picture of a binary tree. There's nothing surprising about that, of course. Just fire up graphviz, say, and in just a few lines you have a png image of your tree. What is surprising is that everything was done in the org file for that post.

Here is the relevant part of the previous post's org file

#+BEGIN_SRC  dot :file tree.png :cmdline -Kdot -Tpng
graph G {
size="2,3";
edge [dir=none, style=line];
node [shape=circle];
5 -- 4;
5 -- 3;
4 -- 9;
3 -- 1;
4 -- 8;
3 -- 6;
}
#+END_SRC

When I put the point inside the block between the #+BEGIN_SRC and the #+END_SRC and typed C-c C-c the code was executed producing the tree

The image of the tree, which appears in the org buffer, is actually a link to the png file that dot produced, of course, and if I type C-c C-x C-v, the picture is toggled off and the link to the file is revealed.

#+results
file:tree.png

When the org file is exported to HTML, the picture is replaced by an <img …> tag.

All this is made possible by Eric Schulte's and Dan Davison's Org-babel, which is now part of the Org-mode distribution. You can find complete documentation for Babel in the Org Manual and there's a nice tutorial at the Worg page.

Babel supports a number of languages as shown on the languages page of the Babel documentation. For example, you can use ditaa to turn ASCII art into nicely rendered diagrams. This

#+BEGIN_SRC ditaa :file tunnel.png :cmdline -r -s 0.65
  +---------+                                                   +---------+
  |         |                                                   |         |
  | Host 1  +<----+                                    +------->+ Host 3  |
  | cBLU    |     |                                    |        | cYEL    |
  +---------+     |                                    |        +---------+
                  |                                    |
                  |                                    |
                  |     +--------+        +--------+   |
                  +---->+        |        |        +<--+
                        |  GW 1  +<--=--->+  GW 2  |
                  +---->+  cRED  |        |  cRED  +<--+
                  |     +--------+        +--------+   |
                  |                                    |
                  |                                    |
  +---------+     |                                    |        +---------+
  |         |     |                                    |        |         |
  | Host 2  +<----+                                    +------->+ Host 4  |
  | cBLU    |                                                   | cYEL    |
  +---------+                                                   +---------+
#+END_SRC

results in this

Finally, here's an example of using gnuplot with Babel. Given this table

Year	Posts
2009	39
2010	10
2011	9

and these gnuplot commands

#+BEGIN_SRC gnuplot :var data=posts :file bar-chart.png
set size .5, 1
set boxwidth .5 relative
set xtics 2008,1,2012
set style fill solid 1.0
plot data using 1:2 with boxes title 'Posts by Year'
#+END_SRC

We end up with

All of these examples produced pictures but Babel supports 29 languages including C, Scheme, elisp, Python, Ruby, and many others that do general computation rather than producing graphics. This is perfect for research because all the data, processing code, and final paper can be kept in a single file—the ultimate example of reproducible research.