Reading a time list and make a plot using matplotlib

Question

I am reading the text file that has two columns in the next format:

20120101 5.6
20120102 5.3
20120103 5.4
...

Where the first column is YYYYMMDD yearmonthday and the second is a magnitude.

Here's what I am doing so far:

file = open('junk.txt','r')
lines = file.readlines()
file.close()

Magnitude=[]
Year=[]

for line in lines:
    p=line.split()

    Year.append(str(p[0]))
    Magnitude.append(float(p[5]))

year = np.array(Year, dtype='datetime64[Y]')
mag=np.array(Magnitude)

fig2 = plt.figure()
ax2 = fig2.add_subplot(1,1,1)
ax2.plot_date(year, Cmag, color='k',linestyle='-',linewidth=2.0)
ax2.set_xlabel('Number of Events')
ax2.set_ylabel('Cumulative Moment')

However the format of the x axis (time) is not correct. I would like to display the time in the format: yyymm in the x axis.

Here's a link with my output (figure):

https://drive.google.com/a/ucsc.edu/file/d/0B3Y1nDlkfy2VNjlBS2FrT0ZRWW8/view?usp=sharing

You can see that time isn't recognized correctly.

could you edit your question to include the output you want versus the output you are getting? this will help to answer your question. — Joseph Farah
– Joseph Farah, Commented Oct 5, 2015 at 3:54
Tried to upload your picture into post, but Google-Drive side settings are not allowing to do that: [Option 1: Set permission to open the file] as either [public] or [anyone who owns the link] — user3666197
– user3666197, Commented Oct 5, 2015 at 5:29

Community · Accepted Answer · 2017-05-23 10:30:40Z

`matplotlib` has a special datetime value ( processing & formatting )

So, a two-step story _{to get 'em PLOT really nice}

Step 1: prepare data into a proper format

from a datetime to a matplotlib convention compatible float for dates/times

As usual, devil is hidden in detail.

matplotlib dates are almost equal, but not equal:

#  mPlotDATEs.date2num.__doc__
#                  
#     *d* is either a class `datetime` instance or a sequence of datetimes.
#
#     Return value is a floating point number (or sequence of floats)
#     which gives the number of days (fraction part represents hours,
#     minutes, seconds) since 0001-01-01 00:00:00 UTC, *plus* *one*.
#     The addition of one here is a historical artifact.  Also, note
#     that the Gregorian calendar is assumed; this is not universal
#     practice.  For details, see the module docstring.

So, highly recommended to re-use their "own" tool:

from matplotlib import dates as mPlotDATEs   # helper functions num2date()
#                                            #              and date2num()
#                                            #              to convert to/from.

Step 2: manage axis-labels & formatting & scale (min/max) as a next issue

matplotlib brings you arms for this part too.

from matplotlib.dates   import  DateFormatter,    \
                                AutoDateLocator,   \
                                HourLocator,        \
                                MinuteLocator,       \
                                epoch2num
from matplotlib.ticker  import  ScalarFormatter, FuncFormatter

aPlotAX.xaxis.set_major_formatter( DateFormatter( '%Y%m' ) )  # OUGHT WORK

Check code in this answer for all details

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Reading a time list and make a plot using matplotlib

1 Answer 1

`matplotlib` has a special datetime value ( processing & formatting )

Step 1: prepare data into a proper format

Step 2: manage axis-labels & formatting & scale (min/max) as a next issue

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matplotlib has a special datetime value ( processing & formatting )

Step 1: prepare data into a proper format

Step 2: manage axis-labels & formatting & scale (min/max) as a next issue

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`matplotlib` has a special datetime value ( processing & formatting )