code: def grid_2d_dy(y_s, y_t):
      dy = y_t - y_s
      return dy
  
file_path: deepgraph/doc/source/tutorials/precipitation.py
imports: import os
  import xarray
  import matplotlib.pyplot
  import numpy
  import pandas
  import deepgraph
  from mpl_toolkits.basemap import Basemap
  from scipy.cluster.hierarchy import linkage, fcluster
label_desc: Use image node_id 2 for calling a global function with example usage: grid_2d_dy(y_s, y_t) and returns: dy