Bugfix: Apply visual mapping for geom aesthetics

**Issue**

Aesthetics `aes` for colors, shapes, size and linestyles
applied to geoms lead to an error.

This code fails.

```python
N = 25
df = pd.DataFrame(np.random.rand(N, 2), columns=list('xy'))
df['area'] = np.random.rand(N) * 100

(ggplot(df, aes('x', 'y', size='area')) +
 geom_point(aes(alpha=0.8)))
```

**Problem**
The line of code that assigns visual aesthetics is
in the wrong place.

**Solution**
Move the line of code so that it also applies to
geom aesthetics.