Here's the line textures I found:
/esoui/art/miscellaneous/wide_divider_left.dds
/esoui/art/miscellaneous/wide_divider_right.dds
/esoui/art/progression/ability_line.dds
/esoui/art/quest/questjournal_divider.dds
So to create the line I do something like (assuming a top level control of MyAddonTLC:
Lua Code:
TopDivider = WINDOW_MANAGER:CreateControl("MyAddonTopDivider", MyAddonTLC, CT_TEXTURE)
TopDivider:SetDimensions(760, 5)
TopDivider:SetAnchor(TOPLEFT, AIResearchGrid, MyAddonTLC, 0, 32)
TopDivider:SetTexture("/esoui/art/quest/questjournal_divider.dds")