ticks rotation does not work sometimes

[kinyatoride]

Description

Hi, thanks for your great work as always.

I found that tick rotation does not work sometimes.

Steps to reproduce

import numpy as np
import proplot as pplt

fig, ax = pplt.subplots()

ax.format(
    xlim=(0, 20), xlocator=np.arange(0, 20, 2),
    xrotation=90,
)

The tick label 10 is not rotated in this case.

Equivalent steps in matplotlib

import numpy as np
import matplotlib.pyplot as plt

fig, ax = plt.subplots()

ax.set_xlim(0, 20)
plt.xticks(np.arange(0, 20, 2), rotation=90)

Proplot version

3.4.3 0.9.7