code: def func(pars, x, data=None):
      a, b, c = pars["a"], pars["b"], pars["c"]
      model = a * np.exp(-b * x) + c
      if data is None:
          return model
      return model - data
  
imports: import matplotlib.pyplot
  import numpy
  from lmfit import Minimizer, Parameters
label_desc: Use image node_id 1 for calling a global function with example usage: func(pars, x, data) and returns: unknown, model