There are 2 Xess functions you will want to use to do this, polyfit and
polycoef.

Take, for example, the following data:

        A       B       C       E       F
-------------------------------------------------
1 |     1       1
2 |     2       2.1
3 |     3       2.9
4 |     4       4.1
5 |     5       4.9
6 |     6       6.2
7 |     7       6.9


To do a least squares fit, you will want to move the cursor to a vacant
square (e.g. C1) and enter the following command,

        @polyfit(A1..A7,B1..B7,1)

The first range is the "x" data and the second range is the "y" data.  The
last number indicates the degree of the polynomial (1 for linear, 2 for
quadratic, etc.).  The polyfit command will then generate the y values
corresponding to the type of polynomial fit you desire.

To get the coefficients of the fit, go to another unoccupied cell (e.g.
E1) and enter the following,
	
        @polycoef(A1..A7,B1..B7,1)

Your spreadsheet now will look like,

        A       B       C       E       F
-------------------------------------------------
1 |     1       1       1.025   0.99642
2 |     2       2.1     2.02142 0.02857
3 |     3       2.9     3.01785
4 |     4       4.1     4.01428	
5 |     5       4.9     5.01071
6 |     6       6.2     6.00714
7 |     7       6.9     7.00357
 

Column C contains the y values of the fitted data points, and column E
contains the coefficients of the fitted polynomial in descending order; 
in a linear fit, these correspond to the slope and intercept of the line.

To plot this, you would select "graph", then "New Graph", then the type of
graph you want (in this case, "Line Graph").  Now you need to select your
data ranges. Go to the graphics window and select "Edit" then "Data
Ranges". Click in the X Range and enter the data range for the x data (you
can also just highlight the data range and it will be echoed in the
window), then click in the Y Range and enter the range for your y data.
Then you want to select "Segment Type" = "No Segment" so only data points
are shown.

Now, to display the fitted curve, go to "Edit", "Data Ranges", then click
on the right arrow "Next" to go to data set 2.  Enter new X-range (same as
the old X-range), and the Y-range corresponding to the fitted data (Column
C). Select "Segment Type" = "Line Segments", then "Apply".  You should
have a graph with data points and a fitted line through them.  If you want
to get a little more sophisticated and display the coefficients on the
graph, goto "Characteristics", "General Characteristics" and select
"Display Legend".  Then, "Edit", "Data Ranges" and enter in the "Legend
Text" field,
	
       0.996x+0.029

and "Apply".

Experiment to get the graph looking the way you want.