Improve barley example

A couple of small improvements/simplifications to the barley example, using new vega features.

Surprisingly it didn't work when I replaced {"value": 103} with {"group": "height"} - and I'm not yet familiar enough with the source to see why.

I think it's also possible to replace the use of the first text mark by an {"scale": "g", "type": "y"} plus some tweaking - maybe this is a sufficiently common use case (labelling facets) that it deserves it's own axis type?