Viewing area is not rescaled to text in shifted references

[hohlraum]

Matplotlib's autoscale does not take artists added by add_artist into account when calculating dataLim. f27da5b introduces an ugly hack to explicitly add text objects to the datalim. It more or less works for cells, but not for references that have been subjected to transformations.

For instance in the code below c.show() works as expected, but top.show() does not.

t1 = core.Text('one', (-10,0), layer=8)
t2 = core.Text('two', (0,5))
t3 = core.Text('three', (-20,5))
t4 = core.Text('four', (10,0))

c = core.Cell('cell')
c.add([t1, t2, t3, t4])

ref = core.CellReference(c, origin=(10,10))

top = core.Cell('top')
top.add(ref)