How to create a logic for Totalising the analog signal?
From what I understand, your problem could be attributed to a mathematical function, that of the integral.
The real problem is that you do not know the mathematical function of the analog signal's time in progress to calculate the plot area - which is equivalent to the sum of your steam flow
From point "a" to "b" you have to break the 24-hour period
into fractions where the analog signal variation is minimal, so that splitting may return you to an area of a rectangle.
If your analog value corresponds to X kg / h of steam, you should memorize the progression of these values - for example every 10 minutes.
The sum of all these values is your total.
This is obviously only one possible solution.
Hope this help you