All chart examples passed the displayBlanksAs parameter as 0 instead of 'gap'.
I added a constants EMPTY_AS_GAP, EMPTY_AS_ZERO and EMPTY_AS_SPAN to the
DataSeries and then change all chart samples to use this new constant.
Fixes#1337Closes#1448
We used to have some kind of wrapper that didn't do much except
forward methods to the real instance. That unnecessary complexity
made it harder to work with the real writer instance.