I am plotting data as a Seaborn heatmap in Python. My data is intrinsically grouped into categories, and I'd like to have lines on the plot to indicate where the groups lie on the map. As a simple example, suppose I wanted to modify this plot from the documentation...
import seaborn as sns; sns.set()
flights = sns.load_dataset("flights")
flights = flights.pivot("month", "year", "passengers")
ax = sns.heatmap(flights, cbar=False)
ax.hlines([3, 6, 9], *ax.get_xlim())