Graphs are updated as data become available, usually around the end of the first week of each month.
If graph has a red background, S&P 500 is fully priced by this measure as of the above date. If graph has a green background, S&P 500 is considered to be undervalued. See Yes, You Can Time the Market for more guidance in interpreting these graphs.
Now that dividend tax rates are about to be lowered, readers might be interested to note how dividend yields have varied with the top tax rates over the past century:
These monthly graphs do not constitute specific investment advice. The data provided are from sources believed to be reliable but cannot be guaranteed.