Function derive_graph 

pub fn derive_graph(
    sessions: &[PiSession],
    title: Option<&str>,
    config: &DeriveConfig,
) -> Graph

Derive a Toolpath Graph from multiple Pi sessions.

Each session becomes one PathOrRef::Path entry in the graph. title becomes graph.meta.title; empty input produces a graph with no paths and graph.id == "graph-pi-empty".