Computes the aspect ratio of the plot

def get_aspect(self, xspan, yspan):
        """
        Computes the aspect ratio of the plot
        """
        if self.data_aspect:
            return (yspan/xspan)*self.data_aspect
        elif self.aspect == 'equal':
            return xspan/yspan
        elif self.aspect == 'square':
            return 1
        elif self.aspect is not None:
            return self.aspect
        elif self.width is not None and self.height is not None:
            return self.width/self.height
        else:
            return 1