I try to use blockdiag in AsciiDoc but the output is somehow broken

[Günter Zöchbauer]

When I include something like 

["blockdiag",target="blockdiag-1.png"]
---------------------------------------------------------------------
blockdiag {
   A -> B -> C -> D;
   A -> E -> F -> G;
}
---------------------------------------------------------------------

I get an blockdiag-1.png which can be displayed properly by Gwenview (a Linux image viewer installed on my system) but the browser (Chrome) and my IDE (WebStorm) can't display it.

Any idea what can cause this problem?

[Günter Zöchbauer]

I forgot to mention that the output is displayed without problems everywhere when I generate the diagram manually on the command line.