Plot Gallons used when records only have total gallons

Here is my setup:

I have a table of Events. Each Event has a date, and a reading of total gallons used.

I want to plot on a chart the number of gallons used since the previous reading. Thus, I am trying to make a field that shows the reading from the prior date, and then use formulas to just do the math and subtract. If I can also get the actual prior date, then I can use duration to find the number of days since the prior reading. More math will give me a gallons per day average, which is the ultimate goal.

I tried Ranking using formula queries, and then more queries to get my info. Unfortunately, I have too many records and get the 'this query will take too long and screw everything up' warning.

Any other ideas on how to get a gallons per day, when the data set I have are records with dates, and total gallons?

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Michael Tamoush
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