from pylab import figure, show, np

t = np.arange(0.0, 2.0, 0.01)
s1 = np.sin(2*np.pi*t)
fig = figure()
ax1 = fig.add_subplot(111)
xstmark=0.5
xenmark=1.0
ystmark=0.5
an1=ax1.annotate(s=' ',xy=(xstmark, ystmark), xycoords='data',
				xytext=(xenmark, ystmark),textcoords='data',
				arrowprops=dict(arrowstyle="<->"))
ax1.annotate(s='tr',xy=(((xstmark+xenmark)/2), ystmark), xycoords='data',fontsize=12.0,textcoords='data',ha='center')
ax1.plot(t, s1)

show()