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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def add_xlabel(self, text=None):
 """ Add a label to the x-axis. """

        x = self.fit.meta['independent']
        if not text:
            text = '$' + x['tex_symbol'] + r'$ $(\si{' + x['siunitx'] +  r'})$'
        self.plt.set_xlabel(text)