Improvements for 03-data_vis_ggplot2

[tavareshugo]

• Add explanation about relationship between geoms and stats.
• Add "Data Tip" about log-transformation.
• Add section on annotations (annotate(), geom_vline(), geom_hline())

[tavareshugo]

Regarding log-transformation:

• perhaps explain absolute versus relative change: https://stats.stackexchange.com/a/48465
• StatQuest video. There's a bit of qPCR at 7:24, which may be confusing for general audience, but still after that it gets generic again with arithmetic using logs. Doesn't talk about how its useful for skewed data though.

[tavareshugo]

Not sure how to explain geoms and stats without it getting confusing.

Here is a stackoverflow post with some comments.

Perhaps using some examples where one does the same graph either specifying geom or stat. And maybe adding an example where changing the default stat is useful, e.g. to overlay points with the mean on top of a boxplot.

Maybe this could be left as optional.