code: def test_stepmodel_linear():
      x, y = get_data()
      stepmod = StepModel(form="linear")
      const = ConstantModel()
      pars = stepmod.guess(y, x)
      pars = pars + const.make_params(c=3 * y.min())
      mod = stepmod + const
  
      out = mod.fit(y, pars, x=x)
  
      assert out.nfev > 5
      assert out.nvarys == 4
      assert out.chisqr > 1
      assert out.params["c"].value > 3
      assert out.params["center"].value > 1
      assert out.params["center"].value < 4
      assert out.params["sigma"].value > 0.5
      assert out.params["sigma"].value < 3.5
      assert out.params["amplitude"].value > 50
  
imports: import numpy
  from lmfit.models import ConstantModel, StepModel
label_desc: Use image node_id 2 for calling a global function with example usage: test_stepmodel_linear() without return types