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I have a series of monotonically increasing values associated with arbitrary dates. In this case, they're meter readings, but could be other values like mileage, fuel usage, etc.

29/05/2015  844.79
01/07/2015  852.20
21/07/2015  856.26

Is it possible to chart this data as a "usage per month" type value rather than an ever increasing value?

I understand it won't be accurate without all the data points, but it should be possible to extrapolate from the points that are there.

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To find the approximate usage, say, in June 2015, I would look at the last reading before this month began and at the first reading after it ended; then pro-rate the use accordingly. This guarantees the usage estimate will be based on an interval that's at least a month long.

I used the Match command to determine the relevant rows for each month, then Indirect to access the data. (Another possibility is Vlookup, but it's a bit too rigid for the present problem.) In the sample below:

  • column C contains the first day of each month (which can be generated with Date), with custom formatting that shows only the month and year.
  • column D has formula =MATCH(C2-1,A:A), etc. This is the last entry in A column preceding the month in column C.
  • column E is =TO_TEXT(C2), etc: this conversion to text is needed so that the chart comes out with month-based labels, and not with some other dates chosen by software.
  • column F contains the logic of computation: for example, F2 is

=(INDIRECT("B"&D3+1)-INDIRECT("B"&D2))/(INDIRECT("A"&D3+1)-INDIRECT("A"&D2))

meaning that one should take the first reading after the month ended, and subtract the last reading from before it began; then divide by the corresponding dates.

+-----------+--------+-----------+-------+------------+-------+
|   Date    | Meter  |   Month   | Match | Text Month | Usage |
+-----------+--------+-----------+-------+------------+-------+
| 4/25/2015 | 830    | May 15    |     2 | May 15     | 0.33  |
| 5/25/2015 | 844.79 | June 15   |     3 | June 15    | 0.20  |
| 7/1/2015  | 852.2  | July 15   |     3 | July 15    | 0.50  |
| 7/21/2015 | 856.26 | August 15 |     5 | August 15  |       |
| 8/3/2015  | 880    |           |       |            |       |
+-----------+--------+-----------+-------+------------+-------+

The above is also available as a shared spreadsheet.

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