TeX Improvements

Thank you for creating this extensive tutorial. A few improvements on using TeX:

1. All you need to do is wrap the text with `$`.
2. You can avoid the double escaping by using raw strings i.e `r'string'`.

So this chunk

```python
from matplotlib import rc
rc('text', usetex=True)

df = pd.DataFrame({"x": np.random.uniform(size=10),
"y": np.random.uniform(size=10)})

ggplot(df, aes("x", "y")) +\
geom_point() +\
labs(x="$\\sum_{i = 1}^n{x_i^2}$",
y="$\\alpha + \\beta + \\frac{\\delta}{\\theta}$")
```

becomes this

```python
df = pd.DataFrame({"x": np.random.uniform(size=10),
"y": np.random.uniform(size=10)})

ggplot(df, aes("x", "y")) +\
geom_point() +\
labs(x=r"$\sum_{i = 1}^n{x_i^2}$",
y=r"$\alpha + \beta + \frac{\delta}{\theta}$")
```