S&P500 relative volume chart

I was looking for the best method to have the S&P 500 volume show up on a chart and I stumbled upon an unusual looking chart that may have some significance.

First, the S&P500 is an index and does not have any volume related directly to the value of the index, it is only a number representative of the group of stocks and their combined value. So I would expect that the volume would be representative of the combined volume of the underlying stocks. Well, the exchange does not send that information with it's feed to the various data processors, like Esignal, stockcharts.com etc. They have to generate this number themselves.

There is an index that I found, $TVOL, which is the total volume of the NYSE. This is not the S&P500 but I think it will be representative of the overall market sentiment.

I found a formula to let me plot this in the lower volume pane of the chart the same as any other volume on a regular equity chart with a moving average, which may be helpful as well. I am not certain yet how this may work on an intraday scale yet.

I have been thinking of trying to track volume at certain times of day to see if the current day is trading higher or lower volume than the same time over the past days. I had not got around to trying to chart this as I was not aware of TVOL yet and thought I would have to come up with my own cumulative market volume indicator, not an easy task.

Which leads me to my interesting chart. I plotted the $TVOL as the primary data on a chart and set it to an hourly chart and this was the result:

Each bar represents one hour, although the first bar is really only 0930h to 1000h as it is set for 0900h-1000h. Worth noting is the last bar as this one is for 1600h - 1700h. Quite a large volume traded after hours, far more than average.

Here is the same chart in a 30 minute period for the last few weeks:
I need to setup a formula to change the colours for each half hour so they are easier to distinguish...but I will only do that if there is any validity or use to the relative time period volume. I think that it will help to differentiate a trending vs ranging day as a high volume start could likely indicate a better moving day...or just a nasty whippy day.

I'll check that out historically a bit then watch it for the next week and see how it correlates. Given the big volume afterhours on Friday I might expect a big day on Monday.

Jeff.