The last figure on this page http://www.vias.org/calculus/07_trigonometric_functions_01_06.html http://www.vias.org/calculus/07_trigonometric_functions_01_0...

The last figure on this page http://www.vias.org/calculus/07_trigonometric_functions_01_06.html http://www.vias.org/calculus/07_trigonometric_functions_01_0... really has that character (in 2D). I wish trigonometry in secondary school had been taught more like this (geometrically) because it would have been good preparation for vector calculus, among other things.