Today the Visio product team released the developer documentation for Visio 2013.
http://msdn.microsoft.com/en-us/library/office/fp161226(v=office.15).aspx
Under the API section in the above article you will find these five important resources:
· Visio 2013 Automation reference
· Visio 2013 ShapeSheet reference
· Visio 2013 Save As Web reference
· Visio 2013 File Format reference
The new file format opens exciting new opportunities, described in detail in this blog article
How to: Manipulate the Visio 2013 file format programmatically
Happy reading!