Reordering a discrete axis in ggplot after generation a lot simpler than one might expect. Rather than converting a character column to a factor (what if the data gets pivoted?), or using one column for the position and one for the labels, you can use xlim or ylim.
> mrna_cols \# desired order \# \[1\] "nav" "cav1" "cav2" "bkkca"
> "shaker" "shal" "shab" "shaw1" \# \[9\] "shaw2" "inx1" "inx2" "inx3"
o_mrna$heatmap_z / # ggplot object within a list
o_mrna$heatmap_z+ylim(mrna_cols).png)