# find min Volatility & max sharpe values in the dataframe (df)
min_volatility = df['Volatility'].min()
max_sharpe = df['Sharpe Ratio'].max()

# use the min, max values to locate and create the two special portfolios
sharpe_portfolio = df.loc[df['Sharpe Ratio'] == max_sharpe]
min_variance_port = df.loc[df['Volatility'] == min_volatility]

# plot frontier, max sharpe & min Volatility values with a scatterplot
plt.style.use('seaborn-dark')
df.plot.scatter(x='Volatility', y='Returns', c='Sharpe Ratio',
                cmap='RdYlGn', edgecolors='black', figsize=(10, 8), grid=True)
plt.scatter(x=sharpe_portfolio['Volatility'], y=sharpe_portfolio['Returns'], c='red', marker='D', s=200)
plt.scatter(x=min_variance_port['Volatility'], y=min_variance_port['Returns'], c='blue', marker='D', s=200 )
plt.xlabel('Volatility (Std. Deviation)')
plt.ylabel('Expected Returns')
plt.title('Efficient Frontier')
plt.show()