Fix disappearing tooltips with lots of segments.

make area obey line.connect_null

Dynamic tickValues

This fixes an issue with tooltips disappearing when graphs are close to the top of the page and have lots of segments, causing them to be bigger than the chart height.

when line.connect_null is set to false areas are disconnected as well as
lines

Closes #451

Data vailidity check

version.

unexpected place, which makes it hard to debug. This patch catches the error
early so as to ease debugging.

Fixed spelling