<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def plot( self, ax=None, title=None, figsize=None, with_candidates=False, candidate_alpha=None, temp_range=None, ):
 """ Plot a model fit. Parameters ax : :any:`matplotlib.axes.Axes`, optional Existing axes to plot on. title : :any:`str`, optional Chart title. figsize : :any:`tuple`, optional (width, height) of chart. with_candidates : :any:`bool` If True, also plot candidate models. candidate_alpha : :any:`float` between 0 and 1 Transparency at which to plot candidate models. 0 fully transparent, 1 fully opaque. Returns ------- ax : :any:`matplotlib.axes.Axes` Matplotlib axes. """

        try:
            import matplotlib.pyplot as plt
        except ImportError:  # pragma: no cover
            raise ImportError("matplotlib is required for plotting.")

        if figsize is None:
            figsize = (10, 4)

        if ax is None:
            fig, ax = plt.subplots(figsize=figsize)

        if temp_range is None:
            temp_range = (20, 90)

        if with_candidates:
            for candidate in self.candidates:
                candidate.plot(ax=ax, temp_range=temp_range, alpha=candidate_alpha)
        self.model.plot(ax=ax, best=True, temp_range=temp_range)

        if title is not None:
            ax.set_title(title)

        return ax