Small tips on the matplotlib tutorial

**Annotating bars**

There is https://matplotlib.org/stable/api/_as_gen/matplotlib.axes.Axes.bar_label.html so that you don't have to roll your own `annotate_bars()` function anymore.

**Boxplots**
Instead of
```python
    ax.boxplot(...)
    ax.set_xticklabels(labels)
```
use
```python
    ax.boxplot(..., tick_labels=labels)
```
