function determineBrushType(brushType, panel) {
	        if (brushType === 'auto') {
	            if (true) {
	                zrUtil.assert(
	                    panel && panel.defaultBrushType,
	                    'MUST have defaultBrushType when brushType is "atuo"'
	                );
	            }
	            return panel.defaultBrushType;
	        }
	        return brushType;
	    }
Merge aixs definition from parallel option (if exists) to axis option.
@inner