Share bar chart styles between journey and bar

We should share the same hover styling between all instances of the same
type of graph. Previously they were specified soley for the journey
module. I've pulled them out into the shared graph styling. Due to
x-axis element being adjacent to the element with class '.bar' we have
to do the text highlighting on a per module basis.
Bar.text should protect from undef and NaN too

Null is not the only value that can end up in text() if there is no
data, more commonly it seems to be undefined. I've written some tests
around this fn and made it a bit more flexible in what it considers to
be 'no data'.