We built a dataflow system for Complex Event Processing (CEP) at one of my previous startups. It included both a visual programming GUI and a non-hideous programming language called SPLASH that allowed one to code nodes of the dataflow graph. The product is now known as the SAP Sybase Event Stream Processor, and this page has an example of SPLASH: http://www.sybase.com/products/financialservicessolutions/co...

(Jon Riecke <http://scholar.google.com/citations?user=lSdHdpUAAAAJ&hl...; did most of the work on the language.)