To export a trendline value to a file you could do something like this:
print( file( "C:\MyFile.txt" ), date, ",", time, ",", TL_GetEndVal( TL_ID ) ) ;
The trendlines that are on the chart will all have ID numbers. If you created them using EasyLanguage or if somebody else did in EL code then you will see something like this
TL_ID = TL_New( ... )
The part to the left of the = will be the variable that you would insert into the parentheses in the first example.