I have tested your code on sample data and it works fine to me.
ax = sns.distplot(x['wr'], kde=False, label='16-4', hist_kws={"alpha": 0.2})
sns.distplot(y['wr'], kde=False, label='17-4', hist_kws={"alpha": 0.2}, ax=ax)
Even when I remove the ax=ax it works, I wonder is it the same to you too. Try it, I hope this will work.
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