Currently, sf::Transformable has:
setScale(const sf::Vector2f& factors);
scale(const sf::Vector2f& factors);
The scale function is conveniently equivalent to setScale(getScale() * factors)
For added convenience sf::Transformable also has:
setScale(float factorX, float factorY);
scale(float factorX, float factorY);
These functions do the same thing, but they construct the factor-vector for you out of factorX and factorY.
Instead of doing scale(sf::Vector2f(x, y)), you can do scale(x, y)
In my opinion, for additional convenience, sf::Transformable should also have:
setScale(float factor);
scale(float factor);
These functions would do the same thing as the other convenience functions, except both parameters of the factor-vector would be factor.
scale(2) would be the same as scale(2, 2)
setScale(2) would be the same as setScale(2, 2)
If people agree, I'd be more than happy to make a pull-request.