expireCache () {
        // do nothing in base, should be overridden by sub-function
        return this;
    }
Expire the internal chart cache. dc charts cache some data internally on a per chart basis to
speed up rendering and avoid unnecessary calculation; however it might be useful to clear the
cache if you have changed state which will affect rendering.  For example, if you invoke
{@link https://github.com/crossfilter/crossfilter/wiki/API-Reference#crossfilter_add crossfilter.add}
function or reset group or dimension after rendering, it is a good idea to
clear the cache to make sure charts are rendered properly.
@returns {BaseMixin}