old_contents: import numpy as np
  
  from .context import sdf
  
  def test_utils_plot_join_line():
      t = {'id':['a','b','c','b'],
           'x' :[1  ,2  ,3  ,4  ],
           'y' :[5  ,6  ,7  ,8  ]}
      x,y = sdf.utils.plot_join_line(t,'id','x','y')
      assert(np.all(np.equal(x,[[2],[4]])))
      assert(np.all(np.equal(y,[[6],[8]])))
  
new_contents: import numpy as np
  
  from .context import sdf
  
  def test_utils_plot_join_line():
      t = {'id':np.array(['a','b','c','b']),
           'x' :np.array([1  ,2  ,3  ,4  ]),
           'y' :np.array([5  ,6  ,7  ,8  ])}
      x,y = sdf.utils.plot_join_line(t,'id','x','y')
      assert(np.all(np.equal(x,[2,4])))
      assert(np.all(np.equal(y,[6,8])))
  
subject: fix test