Add helper to test that the user submitted a ggplot2 about ggcheck HOT 3 CLOSED

Would this be a new function, e.g. is_ggplot(), that would run at the beginning of every *-check chunk, or a test that's incorporated at the beginning of each uses_*() function?

from ggcheck.

Probably both

from ggcheck.

To clarify: probably both in the sense that we should have a public function to test that we're working with a ggplot2 plot. And internally we'll want to add more checks inside each of the ggcheck test functions that ensure we're working with the right object types.

More broadly, we'll also probably want to bring in gradethis and provide some basic feedback, e.g. fail_if_not_ggplot(), where we can be a little more targeted to common ggplot2 problems with our feedback.

from ggcheck.