Question

What is grob and how to create grobs in ggplot?

Answer 1

A grid graphical object (“grob”) is a description of a graphical item. These basic classes provide default behavior for validating, drawing, and modifying graphical objects.

Easier way to create different grob is using rectGrob(), circleGrob() by providing dimensions.

Ex:  

blank_rect = rectGrob(x = unit(0.5,"npc"),y = unit(0.5,"npc"),width = unit(1,"npc"),height = unit(1,"npc"),col = "grey")