[Matplotlib-users] bar plot with datetime x axis

[Christopher Short]

Hi Steve,

When it comes to bar plots, Pandas doesn’t pass a datetime object to matplotlib  (it’s an old issue).

That’s the reason for the error when attempting to format a date  - it isn’t one.

Easiest solution is to plot directly with matplotlib

In your notation below that would be: (nothing - you don’t need to pass the returned axes back to anything)

fmt = mdates.DateFormatter('%Y’)

for i,col in enumerate(df):

	axs[i,0].bar(df.index,  df[col])

	axs[i,0].xaxis.set_major_formatter(fmt)


Though it might be easier to read to omit I, and  zip the two elements together and loop over them like this::

fig, axs = plt.subplots(9,2,figsize=(20,20),sharex=True)

for col, ax in zip(df.columns, axs.flatten()):
	ax.bar(df.index,df[col])
	ax.xaxis.set_major_formatter(fmt)


That will give you horizontal, rather than vertical text as well.

If you need to rotate to fit all the years in use,

	ax.tick_params(labelrotation=30, axis=‘x)   #or whatever angle you want

 Or use a locator to specific what you want

cheers
Chris


> On 9 May 2018, at 11:06 pm, Steve Aplin <aplin.steve at gmail.com> wrote:
> 
> Hello everyone,
> 
> Having a bit of an adventure controlling the display of the x axis when plotting a pandas dataframe or series using pandas plot(), as in df.plot(kind='bar') or s.plot(kind='bar').
> 
> Here's the head of one column:
> 1996-01-01    180.0
> 1997-01-01    159.0
> 1998-01-01    159.0
> 1999-01-01    152.0
> 2000-01-01    121.0
> Name: column A, dtype: float64
> 
> ... and here's its index:
> DatetimeIndex(['1996-01-01', '1997-01-01', '1998-01-01', '1999-01-01',
>                '2000-01-01', ... ],
>               dtype='datetime64[ns]', freq=None)
> I want to show a bunch of subplots, so my code looks like this:
> from datetime import datetime
> import matplotlib.pyplot as plt
> import pandas as pd
> import numpy as np
> import matplotlib.dates as mdates
> %matplotlib inline
> 
> fig, axs = plt.subplots(9,2,figsize=(20,20),sharex=True)
> for i,col in enumerate(df):
>     ax = df[col].plot(kind='bar',ax=axs[i,0])
>    etc.
> 
> ... which gives an x tick value for each index item (i.e. each bar), in full datetime format (YYYY-MM-DD 00:00:00), rotated 90 degrees.
> 
> I don't want full datetime format, I just want the year portion. Googling the issue led me to trying this:
> 
> fig.autofmt_xdate()
> 
> ... which rotates the labels, but still in full datetime format.
> 
> So I tried adding
> formatter = mdates.DateFormatter('%Y')
> ax.xaxis.set_major_formatter(formatter)
> 
> which throws this error:
> ValueError: DateFormatter found a value of x=0, which is an illegal date.  This usually occurs because you have not informed the axis that it is plotting dates, e.g., with ax.xaxis_date()
> ... which is a bit confusing, since the index is a pandas datetimeindex.
> 
> Any ideas about how I could get my xaxis to not show just the year portion?
> 
> Thanks in advance!
> 
> Stephen E. Aplin
> Ottawa Canada
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