The initial grob matrix object with column headings inserted into the
appropriate areas.
Arguments
mat
The grob matrix object the column headings will be added onto.
headings
The headings to be added onto the initial matrix,
in a list with each heading a separate element. The list must have
the same amount of elements as the heading_cols parameter.
heading_cols
Which column positions of the initial matrix the headings
will be placed above, in a list with each heading's column positions a separate element.
The list must have the same amount of elements as the headings parameter.
Can either be numeric indices, or column names of the initial data frame / matrix
passed through grob_matrix.
Default is an empty list. If unaltered, the function will assume the user
wants to apply headings to all columns of the grob_matrix -
in which case only one headings is allowed.
Details
The user must add column headings before adding or altering
any aesthetics.