Fixed rendering of flags located below data series

Previously if the label was below its data point, the connecting line was drawn over the label's text.
Now the connecting line will start from the top of the label in this case.
Ryan/TomW: apply fix for rendering of line of flags located below data series in FlagsSeries.js also
Merge pull request #1524 from youdevise/master

Fixes to the rendering of Flags below their data series