void scroll_history(float delta) {
for(map<NodeID,Node>::iterator it = nodes_begin(); it != nodes_end(); it++) {
Node& n1 = it->second;
if (n1.type() == COMMIT || n1.display_type() == SNAKE_TAIL || n1.display_type() == HEAD)
n1.pos().y += delta;
if (n1.display_type() == SIGN && (lookup(n1.edge(0).target()).type() == COMMIT || lookup(n1.edge(0).target()).oid().type == REF))
n1.pos().y += delta;
}
}
static unsigned size(GraphType SP) {
return std::distance(nodes_begin(SP), nodes_end(SP));
}