The CSS selector or the element which the chart will be set to. D3 selection object can be specified. If other chart is set already, it will be replaced with the new one (only one chart
can be set in one element).
If this option is not specified, the chart will be generated but not be set. Instead, we can access the element by chart.element and set it by ourselves.
Note: When chart is not binded, c3 starts observing if chart.element is binded by MutationObserver. In this case, polyfill is required in IE9 and IE10 becuase they do not support
MutationObserver. On the other hand, if chart always will be binded, polyfill will not be required because MutationObserver will never be called.
Show rectangles inside the chart.
This option accepts array including object that has axis, start, end and class. The keys start, end and class are optional.
axis must be x, y or y2. start and end should be the value where regions start and end. If not specified, the edge values will be used. If timeseries x axis, date string, Date object and
unixtime integer can be used. If class is set, the region element will have it as class.