By default, R Markdown employs
inch as a unit of figure size. You can change it by
hook_figure_unit.
CAUTIONS
fig.width
and fig.height in inches.NULL to the fig.retina
chunk option.hook_figure_unit() changes to the unit of figure size to
millimeters. Note that this hook will not change the default size.
You may change default unit by the first argument,
unit.
Choices are followings: