regression line

[Celine Bonfils <celine@xxxxxxxxxxxxxxxxxxxxxxxxxx>]

Hi Ferret users,

I have got two variables (e.g. temp and prec) for the month of july for 50 
years, over the globe.
I select a region (x=120W:60W,y=15N:45N) in which I select only the points
which fulfill given conditions (cloud, snow).

let var1=temp[x=120W:60W,y=15N:45N]
let var2=prec[x=120W:60W,y=15N:45N]
let p = IF (cloud LT 30 AND snow LT 1) THEN var1
let p = IF (cloud LT 30 AND snow LT 1) THEN var2
so p and q are still a function of lat, lon and time.

And I plot one variable as a function of the other
plot/vs p, q

I would like to compute only one regression line between all these points.

If I use regresst or regressx, I obtain many regression lines.
e.g.:
go regresst
plot/vs p,q                   ! the original scatter plot
plot/vs/line=2/over p,qhat    ! the regression lines in red

Do you have any idea I could compute this unique regression line?
Thank you!
Celine