how do i draw a heart in a graph

Here are a couple more options.

https://www.desmos.com/calculator/qsuvziloxf

https://www.desmos.com/calculator/8am6ghy9hs

and here is a whole heap of fancy ones

http://hubpages.com/hub/Equations-to-Draw-Hearts-on-a-Graphing-Calculator#slide8774580

Here's one possibility......{it's the graph of the cardioid,  r = 8 - 8sin(Θ) }