cell_fun for annotation

[blueskypie]

In the following example, is there a way to display text in certain cells of the annotation x or y based on the cell value? that is, is there a cell_fun for the annotations?

df1=data.frame(a=1:5,b=11:15,x=c('x','x','y','y','z'),y=21:25)

k=columnAnnotation(
  df = df1[3:4]
)

Heatmap(t(df1[1:2]),top_annotation = k)