Can't Specify Data for the X-Axis in Line Graphs?

Most of my spreadsheet work is simple data plotting (usually just x-y plots of values vs. time). Is there some way to specify which column is for the x-axis data (it seems to only care about specifying the y-data)?

Also, there's a serious flaw in how it created a graph. I created a small table, 3 columns by 10 rows. Column A would be the x-axis (time), and coluns B and C would be the data values. I was using this to play with the program to get to learn how to use it.

For column A, I used 0, 2, 4, 6, 8, 10 as the values. When I created the plot, it seemed to be fine, with those values being the x-axis, and lines showing the data from columns B and C. But it plotted the B and C column values against what row in the table they were on, not against the values in column A.

That is, the data values that were in the second row of data should've been at "2" on the x-axis, but were at "1", data on the next row were at "2" on the x-axis instead of "4", etc.

Am I doing something wrong, or what? The graphs themselves aren't really too professional looking as well (can't change line thickness, etc to make them look nice), but that's secondary to being able to specify x and y data, not to mention having it plot the data correctly.