How to derive line element geometrically

Question

I am aware on how to derive the line element in a given co-ordinate system using algebra (jacobian). I have a question which asks to derive it geometrically. In this example we have a 2d plane, in which the $\tilde y$ is the same as $y$, but $\tilde x$ is like the vector $x$ rotated by an acute angle $\theta$. We have the line element in the original $(x,y)$ co-ordinates is $$ds^2=dx^2+dy^2$$ Now how can we show the line element geometrically is this? $$ds^2=d\tilde x^2+d\tilde y^2+2\sin\theta d\tilde x d\tilde y$$ My problem is that I don't know the definition of a line element in the geometric sense. Perhaps a diagram would be helpful as well.