hi ooker
select your polygon, and with 'properties' (f2) → 'shape' → 'edit' you can select the two right corners and move them to the left.
unfortunately there's no snapping in this mode. for a more complex comic layout you shauld use inkscape and import the .svg in scribus. (you're still able to fine tune the frames in scribus, if you have to…)
utnik