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To me it seems actively good to include examples that use well-developed libraries, to show the types of tools and abstractions that one can build on. Restricting oneself to a language's standard library may give the false impression that anything worthwhile requires a large bootstrapping effort.

That said, it would be fun to provide, e.g., a 500-line graphing library to show how one might begin building something like highcharts.

I would think not. Is the interesting part of the problem drawing charts? If not, it makes perfect sense have that abstracted away somewhere else.

I don't think so. Would you rule out things like the python standard library?

Though external library size can be important, does it come into play when talking about your own code structure?

Only if you assume it to be bug free in a way that your code will never expose.