Add Chart.x_axis_dates.
Add GridLine.
Add Chart#vertical_grid_lines.
Add vertical grid lines to Chart.
Move vertical grid lines from Chart to XLabel.
Introduce xy() as syntactic sugar for Coordinate.new().
Change XLabel to use a Label.
Retire XLabelsBuilder.
Remove XLabel#date.
Create GridLine#build_h/vgridline.
Rename *grid_line* to *gridline*.
Add newlines for clarity in generated TeX file.
Create Label#build_x/ylabel.
Improve a comment.
Rename ChartItem#style to ChartItem#bar_style.
Fix code indentation.