Stocks and bonds are positively correlated most of the time, and particularly so in the current market environment, so bonds make poor diversifiers of stock portfolios.
Here is the screenshot from Bank of America Research.
Treasury Bonds, held to maturity, are guaranteed to pay out and the investment is therefore always net positive unless the government defaults. These bonds diversify one's investments and lower volatility.
The graph as presented doesn't make sense unless one is trading a treasury ETF.
So, the practical question I have for my portfolio is what are the correlations with a bond ETF. My findings are in line (generally) with BofA's results over the same period (from Portfolio Visualizer). I wish I had a longer period of ETF data: