The monitor on the left is set to 1680x1050 resolution.

If the monitor has no orientation the xrandr pos settings are 
+1680+0, which is correct.

when I set the rotation to "Left" (or right) The xrandr pos settings need to change to +1050+0 . The settings stay at +1680+0 leaving a large gap in the virtual screen.

This is likely an easy fix. 

While I'm here I suggest adding a X and Y offset of each display, allowing people to control where their displays are in the virtual space. 

Workaround: 

Manually set the xrandr settings.

My Xrandr settings:
(24" lcd with 2 x 22" lcds vertically mounted on either side)

xrandr -o left --output DVI-1 --pos 2972x0
xrandr --output DVI-0 --pos 1052x220
xrandr -o left --output DisplayPort-0 --pos 0x70

My apologies if this is a dupe, I did a fair amount of googling before, and came up empty.