annotate doesn't trigger redraw

Using this example:

``` python
import matplotlib
matplotlib.use("tkagg")
import matplotlib.pyplot as plt
plt.subplot(1,1,1)
plt.annotate("hello", xy=(0.5,0.5))
```

in interactive mode, does not trigger a redraw, and the text is not drawn. After resizing the figure or plotting something else, the text appears. This is with matplotlib 1.5.0.
