Add a label to the y-axis.

def add_ylabel(self, text=None):
        """
        Add a label to the y-axis.
        """
        y = self.fit.meta['dependent']
        if not text:
            text = '$' + y['tex_symbol'] + r'$ $(\si{' + y['siunitx'] +  r'})$'
        self.plt.set_ylabel(text)