def lin_func(p, x):
     """
     The function is internally used by the ODR-fit procedure

     function arguments:
     p: a numpy-array of two elements (slope and y-intersection of a line)
     x: value for which you want to evaluate the function

     The function calculates p[0] * x + p[1] and returns that value.
     """

     m, c = p
     return m * x + c