FAQ.How to draw a nice antialiased line?
Feedback.

Answer:
This code can help you althrough it's not perfect.
You need to set lc to the current color you use.
Example:

Sub drawLine( xs as integer, ys as integer, xe as integer, ye as integer)
  dim intX, intY, count, n, xDiff, yDiff as integer
  dim v, v1, floatX, floatY, xx, yy, xStep, yStep as double
  dim c as color
  
  xDiff=xe-xs
  yDiff=ye-ys
  count=max(abs(xDiff), abs(yDiff))
  xStep=xDiff/count
  yStep=yDiff/count
  xx=xs
  yy=ys
  for n=1 to count
    intX=xx
    intY=yy
    floatX=xx-intX
    floatY=yy-intY
    
    v=(1-floatX)*(1-floatY)*st
    v1=1-v
    c=face.pixel(intX, intY)
    face.pixel(intX, intY)=rgb(v*lc.red+v1*c.red, v*lc.green+v1*c.green, v*lc.blue+v1*c.blue)
    v=floatX*(1-floatY)*st
    v1=1-v
    c=face.pixel(intX+1, intY)
    face.pixel(intX+1, intY)=rgb(v*lc.red+v1*c.red, v*lc.green+v1*c.green, v*lc.blue+v1*c.blue)
    v=(1-floatX)*floatY*st
    v1=1-v
    c=face.pixel(intX, intY+1)
    face.pixel(intX, intY+1)=rgb(v*lc.red+v1*c.red, v*lc.green+v1*c.green, v*lc.blue+v1*c.blue)
    v=floatX*floatY*st
    v1=1-v
    c=face.pixel(intX+1, intY+1)
    face.pixel(intX+1, intY+1)=rgb(v*lc.red+v1*c.red, v*lc.green+v1*c.green, v*lc.blue+v1*c.blue)
    
    xx=xx+xStep
    yy=yy+yStep
  next
End Sub

Notes: PS: st should be 1 and face should be a RGBSurface or a Graphics object.