Perhaps you meant that link: http://www.xamuel.com/inverse-graphing-calculator.php?phrase=Hello+World
[update, concerning your request for explanation]: Once you have a solution to define a plotting formula in the form $\small f(x,y)=0 $ and define the curve to be painted black if that equation is true, then you may combine many such formulas $\small f(x,y) \cdot g(x,y) \cdot h(x,y)=0 $ to draw combined pictures. (the multiplication implements the boolean OR: if any of the components is zero, draw a black point). The key is then to define some primitives, combine them to a letter (shifting coordinates appropriately), and combine that combinations to words, and ...
(Another example is at http://go.helms-net.de/math/fun/pdf/PdFdiscovery_ED.html ; I'd made some fun with an enthusiastic "fermatist" in the newsgroup sci.math one time )
(A bit more explanative is the following; here you can see the composition of the graph by its elements, although the logic is a bit different: click the top-left icon (that with th scribbled face) on http://go.helms-net.de/math/index.htm to see my first version of such a picture, when I was a schoolboy)