Remove SeparatorItem#bar_x_coordinates.
Minor code and comment changes.
Retire XLabelsBuilder#build2.
Add Separator item missing from XLabelsBuilder test.
Move a responsibiliy to BaxXCoordinates.

ChartItem was converting start and end points to a mid-point and
a width, required by the TikZ library.  Moved this conversion to
BaxXCoordinates.
Move x-axis date range responsibility to Chart from ChartItem.