feat(barchart): render charts smaller than 3 lines (#532)

The bar values are not shown if the value width is equal the bar width
and the bar is height is less than one line

Add an internal structure `LabelInfo` which stores the reserved height
for the labels (0, 1 or 2) and also whether the labels will be shown.

Fixes #513

Signed-off-by: Ben Fekih, Hichem <hichem.f@live.de>