How to plot data against specific dates on the x-axis using matplotlib

Question

I have a dataset consisting of date-value pairs. I want to plot them in a bar graph with the specific dates in the x-axis.

My problem is that matplotlib distributes the xticks over the entire date range; and also plots the data using points.

The dates are all datetime objects. Here's a sample of the dataset:

data = [(DT.datetime.strptime('2010-02-05', "%Y-%m-%d"), 123),
        (DT.datetime.strptime('2010-02-19', "%Y-%m-%d"), 678),
        (DT.datetime.strptime('2010-03-05', "%Y-%m-%d"), 987),
        (DT.datetime.strptime('2010-03-19', "%Y-%m-%d"), 345)]

Here is a runnable code sample using pyplot

import datetime as DT
from matplotlib import pyplot as plt

data = [(DT.datetime.strptime('2010-02-05', "%Y-%m-%d"), 123),
        (DT.datetime.strptime('2010-02-19', "%Y-%m-%d"), 678),
        (DT.datetime.strptime('2010-03-05', "%Y-%m-%d"), 987),
        (DT.datetime.strptime('2010-03-19', "%Y-%m-%d"), 345)]

x = [date for (date, value) in data]
y = [value for (date, value) in data]

fig = plt.figure()

graph = fig.add_subplot(111)
graph.plot_date(x,y)

plt.show()

QUESTION SUMMARY:
My situation is more like I have an Axes instance ready (referenced by graph in the code above) and I want to do the following:

Make the xticks correspond to the exact date values. I have heard of matplotlib.dates.DateLocator but I have no idea how create one and then associate it with a specific Axes object.
Get tighter control over the type of graph being used (bar, line, points, etc.)

Just a tip: since your question really is purely about matplotlib and doesn't have anything specific to wxWidgets, it'd might make things easier if you alter your example to use the functions from matplotlib.pyplot and leave the wx stuff out of it. — David Z
– David Z, Commented Aug 15, 2010 at 4:23
@David: Fixed. Thanks, I realized there might be more people who can read matplotlib + pyplot better than matplotlib + wx — Kit
– Kit, Commented Aug 15, 2010 at 4:45

Joe Kington · Accepted Answer · 2010-08-15 15:31:38Z

What you're doing is simple enough that it's easiest to just using plot, rather than plot_date. plot_date is great for more complex cases, but setting up what you need can be easily accomplished without it.

e.g., Based on your example above:

import datetime as DT
from matplotlib import pyplot as plt
from matplotlib.dates import date2num

data = [(DT.datetime.strptime('2010-02-05', "%Y-%m-%d"), 123),
        (DT.datetime.strptime('2010-02-19', "%Y-%m-%d"), 678),
        (DT.datetime.strptime('2010-03-05', "%Y-%m-%d"), 987),
        (DT.datetime.strptime('2010-03-19', "%Y-%m-%d"), 345)]

x = [date2num(date) for (date, value) in data]
y = [value for (date, value) in data]

fig = plt.figure()

graph = fig.add_subplot(111)

# Plot the data as a red line with round markers
graph.plot(x,y,'r-o')

# Set the xtick locations to correspond to just the dates you entered.
graph.set_xticks(x)

# Set the xtick labels to correspond to just the dates you entered.
graph.set_xticklabels(
        [date.strftime("%Y-%m-%d") for (date, value) in data]
        )

plt.show()

If you'd prefer a bar plot, just use plt.bar(). To understand how to set the line and marker styles, see plt.plot() Plot with date labels at marker locations http://www.geology.wisc.edu/~jkington/matplotlib_date_labels.png

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