plotnine gives an error when trying to draw geom_line with non-constant path parameters, but for which lines with less than 2 data points were dropped.

I believe this is because the code below resets the index before removing single point lines

	data.reset_index(drop=True, inplace=True)

	# drop lines with less than two points
	c = Counter(data['group'])
	counts = np.array([c[v] for v in data['group']])
	data = data[counts >= 2]

This then results in iloc indexing out of bounds

	for _, df in data.groupby('group'):
	idx = df.index
	indices.extend(idx[:-1]) # One line from two points
	x = data['x'].iloc[idx]

Example:

from plotnine import *
import pandas as pd
df = pd.DataFrame({"A" : [1,1,1,1,1],
              "B" : [0,0,1,2,2],
              "C" : [1,1,1,1,1],
              "D" : [1,2,3,4,5]})
ggplot(df) + geom_line(aes(x="A", y="C", group="B", color="D"))

Error:

IndexError: index 4 is out of bounds for size 4

This is with version 0.6.0