Select four points. Make their line segments parallel by running.

makeTwoLinesParallel.py

# menuTitle : Make Two Selected Lines Parallel

import math

def sortTuple(tup):
    # sort line coordinates in ascending y order
    tup.sort(key = lambda x: x[1])  
    return tup  

g = CurrentGlyph()

line1_pts = []
line2_pts = []
i = 1
for pt in g.selectedPoints:
    if i < 3:
        line1_pts.append((pt.x, pt.y))
    elif 2 < i < 5:
        line2_pts.append((pt.x, pt.y))
    i +=1
# sort list by y's
line1_pts = sortTuple(line1_pts)
line2_pts = sortTuple(line2_pts)
print("line 1: ", line1_pts)
print("line 2: ", line2_pts)

x1, y1, x2, y2 = line1_pts[1][0]-line1_pts[0][0], line1_pts[1][1]-line1_pts[0][1], line2_pts[1][0]-line2_pts[0][0], line2_pts[1][1]-line2_pts[0][1]
line1_ang, line2_ang = math.degrees(math.atan2(y1, x1)), math.degrees(math.atan2(y2, x2))
avg_ang = (line1_ang + line2_ang)/2
print("line 1 angle: ", line1_ang)
print("line 2 angle: ", line2_ang)
print("average/target angle: ", avg_ang)

print("-----")
print("x1: ", x1)
print("y1: ", y1)
print("x2: ", x2)
print("y2: ", y2)
# desired x for top point for line 1
line1_xdiff =  x1 - (y1 / math.tan(math.radians(avg_ang)))
# desired x for top point for line 2
line2_xdiff =  x2 - (y2 / math.tan(math.radians(avg_ang)))

# factor by which the top shares the xdiff (1/2 for now? should it be split proportionately?)
factor_top = 0.5
# factor by which the bottom shares the xdiff (1/2 for now? should it be split proportionately?)
factor_bot = 0.5

# move points
with g.undo("Make lines parallel"):
    i = 1
    for pt in g.selectedPoints:
        if (pt.x, pt.y) == line1_pts[1]:
            pt.x -= line1_xdiff * factor_top
        elif (pt.x, pt.y) == line2_pts[1]:
            pt.x -= line2_xdiff * factor_top
        elif (pt.x, pt.y) == line1_pts[0]:
            pt.x += line1_xdiff * factor_bot
        elif (pt.x, pt.y) == line2_pts[1]:
            pt.x += line2_xdiff * factor_bot
    
    g.changed()