Extending Simple Monotone Drawings

We prove the following variant of Levi's Enlargement Lemma: for an arbitrary arrangement A of x-monotone pseudosegments in the plane and a pair of points $a,b$ with distinct x-coordinates and not on the same pseudosegment, there exists a simple x-monotone curve with endpoints $a,b$ that intersects every curve of A at most once. As a consequence, every simple monotone drawing of a graph can be extended to a simple monotone drawing of a complete graph. We also show that extending an arrangement of cylindrically monotone pseudosegments is not always possible; in fact, the corresponding decision problem is NP-hard.