python matplotlib how to graph point on line

1import matplotlib.pyplot as plt
2fig = plt.figure()
3ax = fig.add_subplot(111)
4ax.set_xlim(0,10)
5ax.set_ylim(0,10)
6
7# draw lines
8xmin = 1
9xmax = 9
10y = 5
11height = 1
12plt.hlines(y, xmin, xmax)
13plt.vlines(xmin, y - height / 2., y + height / 2.)
14plt.vlines(xmax, y - height / 2., y + height / 2.)
15average = (xmax+xmin)/2
16
17# draw a point on the line
18px = 5 
19plt.plot(px,y, 'ro', ms = 15, mfc = 'r')
20
21# add an arrow
22plt.annotate('Point', (px,y), xytext = (px+0.35, y+0.5), 
23horizontalalignment='right')
24
25# add numbers
26plt.text(xmin - 0.1, y, 'Left', horizontalalignment='right')
27plt.text(xmax + 0.1, y, 'Right', horizontalalignment='left')
28
29plt.axis('off')
30plt.show()