python matplotlib how to graph point on line

import matplotlib.pyplot as plt
fig = plt.figure()
ax = fig.add_subplot(111)
ax.set_xlim(0,10)
ax.set_ylim(0,10)

# draw lines
xmin = 1
xmax = 9
y = 5
height = 1
plt.hlines(y, xmin, xmax)
plt.vlines(xmin, y - height / 2., y + height / 2.)
plt.vlines(xmax, y - height / 2., y + height / 2.)
average = (xmax+xmin)/2

# draw a point on the line
px = 5 
plt.plot(px,y, 'ro', ms = 15, mfc = 'r')

# add an arrow
plt.annotate('Point', (px,y), xytext = (px+0.35, y+0.5), 
horizontalalignment='right')

# add numbers
plt.text(xmin - 0.1, y, 'Left', horizontalalignment='right')
plt.text(xmax + 0.1, y, 'Right', horizontalalignment='left')

plt.axis('off')
plt.show()