Move model files into /input and /output sub-folders.
Move Tex from model/ to process/.
Refactor: extract private methods.
Merge XAxis into Chart.
Add Layout and Builder.
Chart delegates to Layout (wip).
Rename calc_layout to build.
Move Chart#date_to_x_coordinate to Layout.
Pass layout instead of chart to chart_item/separator_item#build.
#build returns elements to be rendered (wip).
Retire ChartItem/SeparatorItem#render.