Ooh, this looks promising:
Think Kit’s Diagram, Fill, and Cut tools make it fast and easy to draw charts, diagrams, graphs, flows, wireframes, and models. Use them to sketch perfect shapes and connectors, and create presentation -ready visual model. […]
Smart Shapes. Founded on years of scientific research, Intention Engine™ is our trademark drawing recognition and rendering system. It detects and corrects dozens of shapes sketched in real- time – all while preserving the uniqueness and feel of hand-drawn lines.
The update is free via the App Store.