How can I change the transparency of a histogram plot in Seaborn using Pairgrid?
With a pandas dataframe, the following should do it without seaborn:

train_df.groupby('Embarked')['Fare'].plot.hist(alpha=0.5) 

If you have to use seaborn you can use FacetGrid without the col and row argument:

g = sns.FacetGrid(train_df, hue='Embarked')
g = g.map(plt.hist, 'Fare', alpha=0.5)
g = g.add_legend()