function set( scale ) {
	/* eslint-disable no-invalid-this */
	if ( !isString( scale ) ) {
		throw new TypeError( format( 'invalid assignment. `%s` must be a string. Value: `%s`.', 'xScale', scale ) );
	}
	// TODO: test for valid scale

	if ( scale !== this._xScale ) {
		debug( 'Current value: %s.', this._xScale );

		this._xScale = scale;
		debug( 'New value: %s.', this._xScale );

		this.emit( 'change' );
	}
}
Sets the x-axis scale.

@private
@param {string} scale - axis scale
@throws {TypeError} must be a string
@returns {void}