Add a label to the x-axis.

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)