I may not be grasping what you are after. How will Excel know when the data is available? From where does Excel get the data once its available?

Generally speaking, you can certainly go ahead and make all the rows you want for the graph with formulas that result in zero until you get other "Input" cells complete. I've done that numerous times. Then, each month I enter the new data for the month and the formulas in the graph data range update, and so does the graph. In my earlier example, go ahead and have Jan - Dec as data series rows in the graph. In this scenario, you will see columns for each row in your data series and as long as the formulas result in zero, then the graph will show zero for that data point.

Am I answering your question?

Matt