[excel] How do I create a chart with multiple series using different X values for each series?

I want to draw several graphs and combine them into one figure. I will explain the problem in an example. Let's say that I want to draw two graphs with these points:

Graph #1 (X and Y are defining a coordinate).

X - Y
_____

1 - 5
2 - 5
5 - 7
9 - 10

Graph #2

X - Y
_____

6 - 8
8 - 12
9 - 7
12 - 8
15 - 11
21 - 11

What I do is create a chart and click on "Select Data". There I create two series and choose X and Y values. However, this doesn't work since it doesn't allow me to choose different X values for different graphs. Although I choose different X values for these two series, the second one is chosen for both. This is how it looks like in the end:

Do you know how to fix this? I'm using Excel 2008 for Mac.