Interesting stuff, but definitely a 101-level course. A more in depth treatment would discuss the trade off between non-data ink and embellishment, which is one of the most difficult problems in good data viz.

Some more specific thoughts:

ALWAYS represent more than one data series if possible: otherwise you're wasting space. But make sure your thesis is still sensible. It's very helpful, for instance, to plot a series of interest against a backdrop of the greater population.

A histogram is NOT the best way to show the average value. A box and whiskers plot shows the average as well as the bulk of the population in a one-dimensional form that can be extended simply for comparison across categories or time.

Also always remember that the data is king. Line smoothing is only a good idea if you're not using it to give the impression that you have more data points than you really do.

All in all their visualizations provide great examples but they could do more to share the nuance that went into designing them.