http://peltiertech.com/Excel/ChartsHowTo/index.html

should help you.

Ivan,

I would use an X-Y Scatter chart.  As an example, go to cell A1 and add your
data for column A.  After adding your column A data, go to cell B1 and add
your data for column B.  When you've completed adding both columns, go back
to cell A1.  Go to the Chart Wizard icon in the Standard Toolbar.  Click on
it and select "X-Y (Scatter)" as the chart type.  The chart sub-type should
be the type at the very top of the dialog box that shows points only.  Hit
the Finish button and you should see your basic chart.

You can connect the lines between the points by clicking once on any of the
points in the plot area.  Then right-click on your mouse, hit "Selected
Object", go to the patterns tab of the dialog box, and select "Line =
Automatic".  All of the points should now be connected with a line.

To put a single line through the middle of those points, again click on any
one of the points.  Right-click on your mouse and go to "Add Trendline".  
Under the "Type" tab of the dialog box click on "Linear".  You should get a
best-fit linear trend running through the chart.

Hope this helps.

----
Regards,
John Mansfield
http://www.pdbook.com