when I define `hue` to color my plot, `map_lower` calls its function more often and looses data compared to the equivalent call without `hue`. Is this a bug or do I make a mistake?

Please see code below

```
import matplotlib.pyplot as plt
import pandas as pd
from scipy import stats
import seaborn as sns

def corrfunc(x, y, **kws):
    r, _ = stats.pearsonr(x, y)
    print(x)
    print(y)
    print(r)

iris = sns.load_dataset("iris")
seax = sns.pairplot(iris, size=2, vars=["petal_width", "petal_length", "sepal_width"])
seax.map_lower(corrfunc)
plt.show()

```

If you change 

```
sns.pairplot(iris, size=2, vars=["petal_width", "petal_length", "sepal_width"])

```

to 

```
seax = sns.pairplot(iris, hue="sepal_length", size=2, vars=["petal_width", "petal_length", "sepal_width"])

```

the code is broken but the plot looks good. So if you run the code without hue corrfunc is called 3 times for the 3 plots in lower. If I add hue="class" to color the plot by the field class the corrfunc is called by lower 8 times or so. I dont understand why coloring with hue has an effect on map\_lower.