import matplotlib.pyplot as plt

import numpy as np

x = np.linspace(-3, 3, 50)

y = 2*x + 1

plt.figure(num=1, figsize=(8, 5),)

plt.plot(x, y,)

ax = plt.gca()

ax.spines['right'].set_color('none')

ax.spines['top'].set_color('none')

ax.spines['top'].set_color('none')

ax.xaxis.set_ticks_position('bottom')

ax.spines['bottom'].set_position(('data', 0))

ax.yaxis.set_ticks_position('left')

ax.spines['left'].set_position(('data', 0))

x0 = 1

y0 = 2*x0 + 1

plt.plot([x0, x0,], [0, y0,], 'k--', linewidth=2.5)

plt.scatter([x0, ], [y0, ], s=50, color='b')

# method 1:

#####################

plt.annotate(r'$2x+1=%s$' % y0, xy=(x0, y0), xycoords='data', xytext=(+30, -30),

textcoords='offset points', fontsize=16,

arrowprops=dict(arrowstyle='->', connectionstyle="arc3,rad=.2"))

# method 2:

########################

plt.text(-3.7, 3, r'$This\ is\ the\ some\ text. \mu\ \sigma_i\ \alpha_t$',

fontdict={'size': 16, 'color': 'r'})

plt.show()