 content for the header.</li>
<li><strong>cells</strong> [<a href='../../class/c3/Selection.Options.html'>c3.Selection.Options</a>] Options to describe the cell contents, styles, events, etc. Use <code>text</code> or <code>html</code> to define the cell contents.</li>
<li><strong>sortable</strong> [Boolean] - Boolean to define if the counter should be user-sortable by clicking on the header.</li>
<li><strong>value</strong> [Function] - A callback to get the <em>value</em> of the cell for sorting or visualiz