How to Correctly Display Dates on the x-axis of a Pandas Line Plot?

Displaying Date on xAxis in Pandas Dataframe Line Plot

Introduction

To display dates on the x-axis of a Pandas line plot, the set_index() method can be used to convert a column containing date values into the DataFrame's index. However, when using matplotlib's DateFormatter function to format the dates, certain issues may arise.

DateFormatter Issue

The image below illustrates the issue with adding DateFormatter to the code:

[Image of line plot with incorrect date formatting]

Notice that the dates start on 5-24 instead of 5-25, and Thursday is incorrectly labeled for 5-25 of 2017. This is often due to timezone-related issues or incompatibilities between pandas and matplotlib datetime utilities.

Incompatibility of Pandas and Matplotlib Datetime Objects

Pandas and matplotlib datetime objects are generally incompatible. When attempting to use a matplotlib dates object on a date axis created with pandas, issues can occur.

One reason for this is that pandas datetime objects are converted to floating-point numbers representing time in days since 0001-01-01 UTC, plus 1. This format differs from matplotlib's datetime format.

Solutions

Using x_compat

To resolve the date formatting issue, the x_compat=True argument can be passed to the plot() method. This instructs pandas not to use its own datetime format, allowing matplotlib's tickers, locators, and formatters to be used.

Plotting Datetime with Matplotlib

Alternatively, if more precise datetime formatting is required, matplotlib can be used for both plotting and formatting. The following code snippet demonstrates this approach:

<code class="python">import matplotlib.pyplot as plt
import matplotlib.dates as dates

# Create the DataFrame
df = pd.DataFrame({'date':['20170527','20170526','20170525'],'ratio1':[1,0.98,0.97]})
df['date'] = pd.to_datetime(df['date'])

# Create the line plot
plt.plot(df['date'], df['ratio1'])

# Set major locator and formatter for the x-axis
plt.gca().xaxis.set_major_locator(dates.DayLocator())
plt.gca().xaxis.set_major_formatter(dates.DateFormatter('%d\n\n%a'))

# Invert the x-axis for chronological order
plt.gca().invert_xaxis()

# Autofmt the x-axis for optimal placement of dates
plt.gcf().autofmt_xdate(rotation=0, ha="center")

# Display the plot
plt.show()</code>

This revised code will generate a line plot with accurately formatted dates on the x-axis.

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