IIUC，像这样的东西？

from matplotlib import pyplot as plt
import numpy as np

b = 2.8977685 * (10 ** -3)

k_range = [i for i in range(3500, 5500)]
results = [b/i for i in range(3500, 5500)]

k = np.array(k_range)
i = np.where(k==4800)

r=np.array(results)
r[i]

plt.plot(results, k_range, marker='x', markevery=[i[0][0]], markeredgecolor='r')
ax = plt.gcf().gca()
ax.annotate("", xy=(r[i], 4800), xytext=(r[i], 4400), arrowprops=dict(arrowstyle="->"))
plt.show()

输出量：

使用matplotlib.pyplot.arrow()：

from matplotlib import pyplot as plt

b = 2.8977685 * (10 ** -3)

k_range = [i for i in range(3500, 5500)]
results = [b/i for i in range(3500, 5500)]

plt.plot(results, k_range)
plt.arrow(6E-7, 4000, 8E-8, 200, ec='red', width = 5E-12, head_width=1E-8)
plt.show()