bug fix, Add more logic to the stacked bar chart rendering, previously, the offset between bar can still be off base on how much less the computed bar height is less than the sperator and stroke width.  For example, a bar with computed height 0 and computed height 3 will both have final height of 0, but the bar with original height 3 would have created an offset space on the screen, so the next drawn bar will need to have less offset so we don't end up with a big blank between bars.
Merge pull request #57 from midoringo8823/bug_fix

bug fix, Add more logic to the stacked bar chart rendering, previously, ...