Instead of plotting red points (black points) over the closing prices to indicate the upper and lower tenth percentiles, we can use matplotlib's fill_between function. This function fills in all the areas between two lines. It takes an optional where parameter that accepts a boolean Series, alerting it to exactly which locations to fill in:

>>> slb_close.plot(color='black', figsize=(12,6))
>>> plt.hlines(y=[lower_10, upper_10], 
               xmin=xmin, xmax=xmax,color='lightgray')
>>> plt.fill_between(x=criteria.index, y1=lower_10,
                     y2=slb_close.values, color='black')
>>> plt.fill_between(x=criteria.index,y1=lower_10,
                     y2=slb_close.values, where=slb_close < lower_10,
                     color='lightgray')
>>> plt.fill_between(x=criteria.index, y1=upper_10, 
                     y2=slb_close.values, where=slb_close > upper_10,
                     color='lightgray')