Push all ultimate elements to the bottom of the graph

In dot rendering, put all ultimate elements (those that nothing further
depends on) on the same rank, meaning the very bottom.

This helps to visualize our ultimate goals more easily and also shows
elements that are probably a requisite to something else that has not
yet been modelled.

techtree-manager/src/tree.rs

@@ -160,6 +160,16 @@ impl Tree {
         Subtree::new_for_element(self, element)
     }
 
+    pub fn iter_ultimate_elements<'a>(&'a self) -> impl Iterator<Item = ElementIndex> + 'a {
+        self.graph.node_indices().filter(|index| {
+            // If there are no incoming edges, then this is an ultimate element!
+            self.graph
+                .neighbors_directed(*index, petgraph::Direction::Incoming)
+                .next()
+                .is_none()
+        })
+    }
+
     pub fn to_dot(&self) -> String {
         let dot = petgraph::dot::Dot::with_attr_getters(
             &self.graph,
@@ -173,9 +183,16 @@ impl Tree {
             &|_g, (_, element)| element.to_dot_node_attributes(None),
         );
 
+        let ultimate_elements: Vec<_> = self
+            .iter_ultimate_elements()
+            .map(|idx| idx.index().to_string())
+            .collect();
+        let ultimate_element_list = ultimate_elements.join("; ");
+
         format!(
             r#"digraph {{
     ranksep=1.2
+    {{ rank=same; {ultimate_element_list}; }}
 {:?}
 }}
 "#,