Python Matplotlib axis is blank for date with trend line

Question

I can correctly plot a trendline with price data but the both X Y axis of date formatting is blank. I am not sure what is messing up this plot configuration for the axis. Here is the Python 2.7 code:

y = df['Close']

# calc the trendline http://stackoverflow.com/questions/26447191/how-to-add-trendline-in-python-matplotlib-dot-scatter-graphs

l = []
for t in df['Time']:
    datetime_object = datetime.datetime.strptime(str(t), '%H:%M')
    print datetime_object.hour
    print datetime_object.minute
    l.append((3600 * datetime_object.hour + 60 * datetime_object.minute))
x = l
z = np.polyfit(x, y, 1)
p = np.poly1d(z)
fig = plt.figure()
ax = fig.add_subplot(111)

#http://stackoverflow.com/questions/17709823/plotting-timestamps-hour-minute-seconds-with-matplotlib
plt.xticks(rotation=25)
ax = plt.gca()
ax.set_xticks(x)
xfmt = md.DateFormatter('%H:%M')
ax.xaxis.set_major_formatter(xfmt)

ax.plot(x, p(x), 'r--')
ax.yaxis.set_major_formatter(mtick.FormatStrFormatter('%3.4f')) #http://stackoverflow.com/questions/29188757/matplotlib-specify-format-of-floats-for-tick-lables
plt.show()

Also, df['Close'] would have value samples of:

114.684
114.679

df['Time'] would contains sample values:

23:20
23:21

Answer 1

Update: I found the source of your problem.

In addition to the below problem you incorrectly copied the answer to the linked question.

You wrote: ax.yaxis.set_major_formatter(mtick.FormatStrFormatter('%3.4f')) You need: ax.yaxis.set_major_formatter(FormatStrFormatter('%3.4f'))

See updated graph:

https://i.sstatic.net/4fhxY.jpg

In your code, you begin axis changes before you have actually plotted anything.

If you move your ax.plot(x, p(x), 'r--') to just below your add_subplot line this will work:

import numpy as np
from matplotlib import pyplot as plt
import datetime

import matplotlib
from matplotlib.ticker import FormatStrFormatter

df = pandas.DataFrame()
df['Time'] = pandas.Series(['23:2','22:1'])
df['Close'] = pandas.Series([114.,114.])

y = df['Close']

# calc the trendline http://stackoverflow.com/questions/26447191/how-to-add-trendline-in-python-matplotlib-dot-scatter-graphs

l = []
for t in df['Time']:
    datetime_object = datetime.datetime.strptime(str(t), '%H:%M')
    print datetime_object.hour
    print datetime_object.minute
    l.append((3600 * datetime_object.hour + 60 * datetime_object.minute))
x = l
z = np.polyfit(x, y, 1)
p = np.poly1d(z)
fig = plt.figure()
ax = fig.add_subplot(111)
#Added:
ax.plot(x, p(x), 'r--')

#http://stackoverflow.com/questions/17709823/plotting-timestamps-hour-   minute-seconds-with-matplotlib
plt.xticks(rotation=25)
ax = plt.gca()
ax.set_xticks(x)
xfmt = md.DateFormatter('%H:%M')
ax.xaxis.set_major_formatter(xfmt)

# REMOVED: ax.plot(x, p(x), 'r--')
# Changed: ax.yaxis.set_major_formatter(mtick.FormatStrFormatter('%3.4f')) 
ax.yaxis.set_major_formatter(FormatStrFormatter('%3.4f'))
#http://stackoverflow.com/questions/29188757/matplotlib-specify-format-of-    floats-for-tick-lables
plt.show()