### flipping a graph to plot upside down

Because in my special application positive values on my y-axis increase downwards instead of upwards.

So while normally you would do this:

```
x,y = var('x,y')
f(x,y) = x^2 + y^2
contour_plot(f, (x,0,1), (y,0,1)).show()
```

(where y plots from smaller numbers to bigger ones) I instead want to do this:

```
x,y = var('x,y')
f(x,y) = x^2 + y^2
contour_plot(f, (x,0,1), (y,1,0)).show()
```

where I expect the same picture, just flipped upside down. Except, this bombs! How can i get my desired result? the axis should be properly labeled, too.