x-axis ticks change location when data is None - matplotlib

Question

I plotted a list using:

fig = plt.figure(1)
ax1 = fig.add_subplot(111)
ax1.plot(range(samples_per_ant[0]), new_y[:samples_per_ant[0]], '-o', linewidth=3)

The result is that the ticks are in the right positions, equally spaced in between and from the plot's frame (i can't add a picture to share).

But when i want to change the ticks i used:

ax1.set_xticks(range(samples_per_ant[0]), case[:samples_per_ant[0]])

since the last few elements in list "new_y" are "None", such as

new_y = [1, 2, 3, ... , None, None, None]

it looks like it "stretched" the x-axis a bit and the ticks got to the right edge of the frame (no space from the edge, as supposed to be).

How can i plot it and keep the ticks in place?