Combining graphs - horizontal axis issues

Hi all,
I'm trying to superimpose multiple graphs in excel. The y-axis is tumor volume for all graphs, and the x-axis should be days from zero. The tumors were not measured every day, and each study was measured on different days (i.e. for one study days 10, 15, 20, and the next study days 18, 24, 30). I have each graphed individually, but when I superimpose them it seems to be reading those days not as days but as labels for 3 time points. I mean, if I have those two examples listed above charted on top of each other (using the copy/paste method) it lines up the 10 and 18 on the same grid line, 15 and 24, 20 and 30. Is there a way to change the horizontal axis or the way excel is reading those numbers so that it will see them as days (0 - 100) and not as points 1, 2, 3? I need to present this sometime soon, and I have over 130 of these that I need to superimpose on top of each other!
Thanks!
Angela

please post an example file, it will be easier to solve your issue