Add grids to ggplot.
grids(axis = c("xy", "x", "y"), color = "grey92", size = NULL, linetype = NULL)axis for which grid should be added. Allowed values include c("xy", "x", "y").
grid line color.
numeric value specifying grid line size.
line type. An integer (0:8), a name (blank, solid, dashed,
dotted, dotdash, longdash, twodash). Sess show_line_types.