label = r'$frac{d^2y}{dx^2}$'

import numpy as np
import matplotlib.pyplot as plt

x = np.linspace(1, 10, 10)
y = x

def f(x, y, n):
    """ """
    fig, ax = plt.subplots()
    if n == 1:
        label = r'$frac{dy}{dx} = 0$'
    else:
        numerator = 'd^{}y'.format(n)
        denominator = 'dx^{}'.format(n)
        # label = r'$frac{}{}$'.format(numerator, denominator)
        # label = '$frac{}{}$'.format(numerator, denominator)
        # label = '$frac{numerator}{denominator}$'
        label = r'$frac{numerator}{denominator}$'
    ax.scatter(x, y, c='r', marker='.', s=5, label=label)
    ax.legend(loc='upper left')
    plt.show()
    plt.close(fig)

f(x, y, n=1)
f(x, y, n=2)