Sort density scatter

[ElliottKasoar]

Proposed by @JonathanSchmidt1:

                if density:
                    order = np.argsort(density)
                    sample_x = [sample_x[i] for i in order]
                    sample_y = [sample_y[i] for i in order]
                    density = [density[i] for i in order]

Originally posted by @ElliottKasoar in #340

But this may interfere with @joehart2001's plans to visualise from density plots