Add new version of Chart (WIP).
Add SeparatorItem#calc_layout.
Add ChartItem#calc_layout.
Add Chart#y_axis_length.
Remove unused variable.
Add Chart#x_axis_labels.
Replace Chart with Chart2 (WIP).
Retire ChartBuilder.
Rename Chart2 to Chart.
Rename Chart#chart_items tp Chart#items.
Rename x_length to x_axis_length.
rename y_length to y_axis_length.
Rename x_labels to x_axis_labels.