I still haven't found any setting to straighten the trees, but...
Under the heading of render buffers, I have found quite a few things.
Those that don't work:
Default, Per Preview, Per Model Per Preview
Those that do work:
Horizontal Stack, Vertical Stack, Horizontal Per Model/Strand, Overlay - Centered, Overlay - Scaled, Per Model Default
The others do other things that don't allow for bars or morph to present lines.
Note - Morph only draws the line at the start. After that is more of a nice face in of each new color.