Real nice! I made it to the 125 kg level.
There’s two little things I’d have done different: First, the line’s velocity curve is unnatural. A pendulum reaches its fastest at the bottom of the arc, but yours seems to get a kick at the edges right when it reverses direction (when the eye expects velocity to be at its minimum and gradually increasing).
Second, a finer point, the Game Over and Hello screens require a mouse click, but the game is otherwise playable with space bar alone – space ought to work there too.
Great work on the whole! This really feels like something we’d have had on the school PC back in the 80s. :)