### Vector Field example fails

The following example fails with an error message in the last line.

This is an example taken from the "manifold.pdf" reference file, Page 337

I'm using SageMath 8.1 on Windows Native.

```
sage: S2 = Manifold(2, 'S^2')
sage: U = S2.open_subset('U') # the open set covered by spherical coord.
sage: XS.<th,ph> = U.chart(r'th:(0,pi):\theta ph:(0,2*pi):\phi')
sage: R3 = Manifold(3, 'R^3')
sage: X3.<x,y,z> = R3.chart()
sage: F = S2.diff_map(R3, {(XS, X3): [sin(th)*cos(ph),sin(th)*sin(ph), cos(th)]}, name='F')
sage: F.display() # the standard embedding of S^2 into R^3
sage: v = XS.frame()[1] ; v # the coordinate vector d/dphi
sage: graph_v = v.plot(chart=X3, mapping=F, label_axes=False)
sage: graph_S2 = XS.plot(chart=X3, mapping=F, number_values=9)
sage: graph_v + graph_S2
```

Moreover, each individual plot does not generate an error, but I don't see any plot, just an empty space!