domainer.include() added, used on bar plots

Fix #573: `Domainer`s have no way to say "include this point in the domain", so bar plots failed to include `0`. This branch adds a `domainer.include(n)` that will ensure that `n` is in the output domain.

Though you can add whatever points you want, bar plots now include their `_baselineValue`, which is typically `0`.

Close #573.
This will gradually add more and more inclusions each time the baseline value is changed. We'll need to remove the previous baseline value from the domainer whenever the baseline value changes...