Computes the aspect ratio of the plot

def get_aspect(self, xspan, yspan):
        """
        Computes the aspect ratio of the plot
        """
        if isinstance(self.aspect, (int, float)):
            return self.aspect
        elif self.aspect == 'square':
            return 1
        elif self.aspect == 'equal':
            return xspan/yspan
        return 1