Add the % symbol to the format when using percentage statistical variables

```python
import numpy as np
from lets_plot import *

LetsPlot.setup_html()

np.random.seed(42)

data = {'x': np.random.randint(10, size=3000)}
ggplot(data, aes(x='x', y='..sumpct..')) + geom_bar()
```

Output:

Would be great to see % in tick labels and tooltips