How to fit data with a straight line

[Yang Haijun <navyang@xxxxxxxxxxxxxxxxxxxx>]

Dear ferret users,

I have a time-series data, say Temperature with 40 years, now I want to fit this data with a 
straight line, that is, I want to get this line:

  Y = a + b*t
  
 based on Temperature observations. Here Y is temperature, t is time step. How do I get 'a' and 
'b'? Is there a function to do this in ferret?
 
 Thank you very much,
 
 Haijun Yang