(Partial) rewrite of fig7 code

I've rewritten the figure 7 code to use a better style:

* create a data frame where the columns are variables and the rows are observations
* use vectorised base graphics commands

It's not quite the same as the original (I didn't preserve the sizes etc), and I didn't quite finish the line drawing, but it should be simple to complete.

You'll notice that the code is considerably shorter ;)