Here is an example drawing a red default thickness (4) line diagonally:
local f = CreateFrame("Frame")
f:SetPoint("CENTER", 0, 100)
f:SetSize(100,100)
local l = f:CreateLine()
print(l)
l:SetColorTexture(1,0,0,1)
l:SetStartPoint("TOPLEFT",10,10)
l:SetEndPoint("BOTTOMRIGHT",10,10)
f:Show()
