show label on matplotlib example

![plot_matplotlib_hist_1](https://f.cloud.github.com/assets/202816/245012/b2dc8472-8a74-11e2-9cb1-341ce7511c9f.png)
![root_plot_matplotlib_hist_1](https://f.cloud.github.com/assets/202816/245016/b5d68ae2-8a74-11e2-8f70-134878d74884.png)