I realise the question was asked several years ago - I came across it on a search - the problem with SIN() and COS() in PICBASIC is that they return a twos complement number. Also they deal with "binary radians" (not the same as trig radians) where 0 is 0 degrees/radians and 255 is 360 degrees or 2PI radians.
Here is my solution to displaying a barchart of the sine wave function 128+sin(i)
(using http://www.picbasic.co.uk/forum/showthread.php?t=2359 to display):

Value var word 'must use word variable for BARGRAPH
twoC var byte 'must use byte variable
include "LCDbar_INC.bas"
'BARgraph Value, Row, Col, Width, Range, Style

'........... I use in a loop and increment i etc............

twoC = sin(i)
if (twoc & %10000000)=0 then PosS 'else negative 2's complement
' value = 127-((twoc ^ %11111111) +1) 'restore negative value
value = 126 - (twoc ^ %11111111) 'I wanted a 128 offset
goto ShowS
PosS: value = 128 + twoc
ShowS:
@ BARgraph _Value, 1, 0, 16, 256, lines

'............ Fiddle/pause and go round the loop..........

Peter Finch