Gscope is a software oscilloscope library build using the GTK and GNOME toolkits. Gscope can be used to monitor, display and debug various types of "real-time" data. Additionally, the scope can be used for graphing file data that is in a tuple format.
The Gscope Project Page.
Read the online Gscope manual and the Gscope API.
The Gscope FAQ.