Plotting Galactic Longitude from 180 to -180
Assuming

x, y = rrl_pm.l, rrl_pm.b


The transformation you want is 

xx = [(q+180)%360 - 180 for q in x]


Add 180, do modulo 360, then subtract 180.

Then set your limits

ax.set_xlim(180, -180)






import matplotlib.pyplot as plt

x = (0, 10, 20, 40, 80, 160, 200, 280, 320, 340, 350)
y = (0, 10, 20, 30, 40, 50,  -50,  -40, -30, -20, -10)

fig = plt.figure(figsize=(8,5))
ax  = fig.add_subplot(1,1,1, aspect='equal')

ax.scatter(x, y)

ax.set_xlim(0, 360)
ax.set_ylim(-90., 90.)

plt.title('Old', fontsize=18)

plt.show()

xx = [(q+180)%360 - 180 for q in x]  # DO THIS!

fig = plt.figure(figsize=(8,5))
ax  = fig.add_subplot(1,1,1, aspect='equal')

ax.scatter(xx, y)

ax.set_xlim(180, -180)
ax.set_ylim(-90., 90.)

plt.title('New', fontsize=18)

plt.show()