A nicer way to format code blocks in Markdown is like this:

~~~
digraph G {
subgraph clusterA {1}
subgraph clusterB {a b c}
1 -> a -> 1
1 -> b -> 1
1 -> c -> 1
}
~~~

To do this, you begin the block with a line ~~~, and end it with a line ~~~ as well.

To see how I did this, click on the gear for this comment (which becomes visible when you hover the mouse over top right corner of the comment panel), and click View Source.