Adds custom annotations to a single panel on the ggbrain plot

Usage

annotate_panel(x = "middle", y = "middle", slice_index = NULL, ...)

x: the x position of the annotation. If numeric, it is assumed to be the pixel position along the x axis (e.g., 26). In addition, convenience values of 'left', 'right', or 'q[1-100]' can be used to look up the left-most, right-most, or quantile-based positions along the x axis.
y: the y position of the annotation. If numeric, it is assumed to be the pixel position along the y axis (e.g., 26). In addition, convenience values of 'left', 'right', or 'q[1-100]' can be used to look up the left-most, right-most, or quantile-based positions along the x axis.
slice_index: the slice number to which this annotation is added. These are numbered in the wrapping order from patchwork::wrap_plots, which will normally go from top-left to bottom-right.
...: Additional parameters passed to ggplot2::annotate such as label or geom

a ggb object with the relevant annotations field and an action of "add_annotations"

Note that this only handles a single annotation on a single panel!