def format_coord(x, y):
try:
z = # get value depending on x,y, e.g. via interpolation on grid
# I can't fill this because the kind of data is unknown here
return "x: {}, y: {}, z: {}".format(x,y,z)
except:
return "x: {}, y: {}".format(x,y)
ax.format_coord = format_coord