object 'toydata_with_na' not created in visible code

[drmowinckels]

Readme code fails when trying to plot the 'toyota_red' object. Make sure that the object is created with visible code.

part of openjournals/joss-reviews#4740

[cosimameyer]

Thanks for the helpful suggestion - I addressed the issue in PR #25 and included the code chunk to generate toydata_with_na 😊