fix for #652 get_max_drawdown_underwater 🔧

Author: f0ster

pyfolio/timeseries.py

@@ -890,7 +890,7 @@ def get_max_drawdown_underwater(underwater):
         The maximum drawdown's recovery.
     """
 
-    valley = underwater.idxmin()  # end of the period
+    valley = underwater.index[underwater.values.argmin()] # end of the period
     # Find first 0
     peak = underwater[:valley][underwater[:valley] == 0].index[-1]
     # Find last 0