function filterOutUnkownValues(d) {
            return d.map(layerEls => {
                for (let i = 0; i < layerEls.length; i++) {
                    layerEls[i] = getValOrDefaultToZero(layerEls[i]);
                }
                return layerEls;
            });
        }
Filter out unkown stacks/values in the bar layers
@param {Object[]} d
@return {Object[]} filteredData
@private