CODEWITHSUNDEEP
pythondatetime
Calaf
Calaf

Reputation: 10847

Using datetime objects

Part 1 below illustrates using datetime objects to plot a curve.

Part 2 illustrates using floats to plot a set of segments.

Part 3 merely blends Parts 1 & 2, but it fails. Why?

import datetime
import numpy as np
import matplotlib.pyplot as plt
from matplotlib import collections  as mc

#----------------Part 1----------------
d0 = datetime.datetime(2001, 1, 1)
d1 = datetime.datetime(2002, 1, 1)
d2 = datetime.datetime(2003, 1, 1)
d3 = datetime.datetime(2005, 1, 1)
d4 = datetime.datetime(2007, 1, 1)
d5 = datetime.datetime(2009, 1, 1)

date  = [ d0, d1, d2, d3, d4, d5 ]
price = [ 5, 4, 6, 7, 3, 8 ]

plt.plot(date, price)
plt.show()

#----------------Part 2----------------
lines = [ [ (0.5, 1.2), (1.1, 1.3) ],
          [ (2.2, 2.8), (3.1, 4.2) ],
          [ (1.9, 2.9), (0.2, 1.4) ] ]

lc = mc.LineCollection(lines)
fig, ax = plt.subplots()
ax.add_collection(lc)
ax.autoscale()
ax.margins(0.1)
plt.show()

#----------------Part 3----------------
lines = [ [ (d0, 1.2), (d1, 1.3) ],
          [ (d2, 2.8), (d3, 4.2) ],
          [ (d4, 2.9), (d5, 1.4) ] ]

lc = mc.LineCollection(lines)
fig, ax = plt.subplots()
ax.add_collection(lc)
ax.autoscale()
ax.margins(0.1)

plt.show()

Update

The line 

lc = mc.LineCollection(lines)

in Part 3 bails out with the error:

Traceback (most recent call last):
  File "datetime-difficulty.py", line 37, in <module>
    lc = mc.LineCollection(lines)
  File "/lib/python/matplotlib/collections.py", line 897, in __init__
    self.set_segments(segments)
  File "/lib/python/matplotlib/collections.py", line 906, in set_segments
    seg = np.asarray(seg, np.float_)
  File "/lib/python/numpy/core/numeric.py", line 235, in asarray
    return array(a, dtype, copy=False, order=order)
TypeError: float() argument must be a string or a number

Upvotes: 4

Views: 595

Answers (1)

Zachary Cross
Zachary Cross

Reputation: 2318

It looks like you are using matplotlib, so you'll need to convert your dates into floats in order for your plots to work. Thankfully, matplotlib supplies the date2num() function. Run all of your dates through that function, and matplotlib should be able to give you a meaningful x-axis (depending on formatter / locator).

Upvotes: 2

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