I have occasionally recorded usage data from a metering system (water pumps or gas tank), with the first two columns below: (Date, reading)

DateTime meter_reading consumed time_diff daily_average_equivalent
7/2/12 9:00 AM 500 -
7/3/12 8:00 AM 512 12 (B3-B2) (A3-A2) (consumed/time diff)
7/7/12 5:00 PM 520 8 (B4-B3) (A4-A3)
7/14/12 8:30 AM 531 11 ... ...
7/27/12 9:30 AM 551 20
8/12/12 12:00 PM 600 49


I then calculated consumption (3rd col) and then can calculate time difference (4th), and daily average consumption equivalent (5th) to get comparable measures.

At this point I would like to draw a chart to ease reading.

I can easily do an X/Y graph with time scale and Column B.
I get the growing meter reading line (value never reducing).
This is fine but I would like to represent column C (kind of flatten down the rising line to delta values).
How can I do this?

I picked a column graph for column C, but I would like to show the consumed value NOT with a thin column in reading date, but with a wider flat area from previous to current reading (meaning that I assume the consumption was evenly distributed between the readings).
Is there a way to do this graph starting from data collected as above, without the need to create an expanded data table with each day by day interpolated values?

Thanks