Address comments for cagan_ree lecture (#387)

* update cagan ree

* remove extra annotations in the code

* remove double spaces

* remove figsize

* Revert "remove figsize"

This reverts commit c2afe22.

* improve code comments based on feedback

* update feedback for the second section

* update labels and descriptions

* update request on `the`

* update graph function and set global dpi

* distinguish the names of the two plotting functions

* switch to tuple whenever possible

* revert changes on dpi settings

Author: HumphreyYang