FYI for the input "Graph a cartoid and explain it" I get a correct definition and some interesting facts/properties (in spite of the fact that I mis-remembered the name of the object I had in mind; it should have been "cardioid"). But then it tries to provide a plot, and the plot has an error:

``` Error Please check your input: Undefined variable heta ```

Looks like there's a "theta" that's being incorrectly truncated, or something to that effect.

I refreshed a few times and found that I consistently saw this same error.

I was able to replicate your error and pushed a fix, it should be working pretty reliably now for equations that include theta. Let me know if it works for you!