Stacked columns don't support spacing around the zero line

[novemberborn]

In a stacked column chart, if the stack contains negative and positive columns, the stackSpacing does not apply between the columns immediately below and above the zero line.

We could consider taking half the spacing is away from each column. Note though that the zero line is considered to be 1px high, and this may look strange when other stacks in the same chart contain only negative or positive columns.