The answer to this question appears relevant to my problem, however, it applies for ax.bar() instead of ax.vlines.
Matplotlib DateFormatter for axis label not working
The code below works with ax1.vlines(x, l, h, colors='k') and ax2.vlines(x, 0, v, colors='k') to plot vertical price and volume bars in a stock chart. But the horizontal axis is defined by a numpy array x = 0,1,2,3, ... etc. I have datetime objects in array d but if change to ax1.vlines(d, l, h, colors='k') and ax2.vlines(d,0,v,colors='k') then it throws an error. Thus d is defined but not used in the code below (it won't work using d but it works using x in the referenced code lines).
import datetime
import numpy as np
import pandas as pd
import matplotlib.pyplot as plt
df = pd.read_csv(PATH+ticker+EXT, usecols=[0,2,3,4,5], header=None,
engine='python',skiprows=skr,skipfooter=skf)
d = pd.to_datetime(df[0]) # numpy array date
h = df[2].values # numpy array high
l = df[3].values # numpy array low
c = df[4].values # numpy array close
v = df[5].values # numpy array volume
x = np.arange(len(d))
# Draw Chart to White Background
ax1_y_label = ticker
fig1 = plt.figure()
fig1.set_size_inches(WIDE,TALL)
fig1.set_dpi(DTPI)
fig1.autofmt_xdate()
ax1 = plt.subplot2grid((5,4), (0,0), rowspan=4, colspan=4)
ax1.set_ylabel(ax1_y_label)
ax1.grid(True)
ax1.vlines(x, l, h, colors='k')
ax1.hlines(c, x, x+0.3, color='k')
ax2 = plt.subplot2grid((5,4), (4,0), sharex=ax1, rowspan=1, colspan=4)
ax2.set_ylabel(ax2_y_label)
ax2.grid(True)
ax2.vlines(x, 0, v, colors='k')
ax1.spines['top'].set_visible(False)
ax1.spines['right'].set_visible(False)
ax2.spines['right'].set_visible(False)
plt.setp(ax1.get_xticklabels(), visible=False)
plt.setp(ax1.get_yticklabels(), visible=False)
plt.setp(ax2.get_yticklabels(), visible=False)
plt.subplots_adjust(hspace=.01)
