function preprocessForPiecewise(thisOption) {
	        var pieceList = thisOption.pieceList;
	        thisOption.hasSpecialVisual = false;

	        zrUtil.each(pieceList, function (piece, index) {
	            piece.originIndex = index;
	            // piece.visual is "result visual value" but not
	            // a visual range, so it does not need to be normalized.
	            if (piece.visual != null) {
	                thisOption.hasSpecialVisual = true;
	            }
	        });
	    }
Give order to visual types, considering colorSaturation, colorAlpha depends on color.

@public
@param {(Object|Array)} visualTypes If Object, like: {color: ..., colorSaturation: ...}
                                    IF Array, like: ['color', 'symbol', 'colorSaturation']
@return {Array.<string>} Sorted visual types.