A normal graphing calculator, you enter an equation and get a curve. An inverse graphing calculator goes the other way: from curve to equation. The current version accepts curves in the form of English letters. So it can generate equations whose graphs appear to spell things out, like “Hello World”.
The calculator is here: Inverse Graphing Calculator
Here it is with a phrase already entered: Hello World
Remember the way a graph of an equation works: the graph is the set of points (x,y) which satisfy the equation. For example, the point (1,2) lies on the curve “y=2x” because the y coordinate (2) is double the x-coordinate (1), hence y=2x is true when you plug in x=1,y=2.
The IGC is in beta version. Future versions will have a lot more features 
This is only the beginning
FURTHER READING
Ambiguities in Mathematics
One Equation to Rule Them All
The Japanese Restaurant Name Generator