function setXValue( fcn ) {
	/* eslint-disable no-invalid-this */
	if ( !isFunction( fcn ) ) {
		throw new TypeError( format( 'invalid assignment. `%s` must be a function. Value: `%s`.', 'xValue', fcn ) );
	}
	this._xValue = fcn;
}
Sets the x-value accessor.

@private
@param {Function} fcn - accessor function
@throws {TypeError} must be a function