matplotlib: Creating two (stacked) subplots with SHARED X axis but SEPARATE Y axis values

Question

I am using matplotlib 1.2.x and Python 2.6.5 on Ubuntu 10.0.4. I am trying to create a SINGLE plot that consists of a top plot and a bottom plot.

The X axis is the date

Answer 1

There seem to be a couple of problems with your code:

1. If you were using figure.add_subplots with the full signature of subplot(nrows, ncols, plotNum) it may have been more apparent that your first plot asking for 1 row and 1 column and the second plot was asking for 2 rows and 1 column. Hence your first plot is filling the whole figure. Rather than fig.add_subplot(111) followed by fig.add_subplot(212) use fig.add_subplot(211) followed by fig.add_subplot(212).

2. Sharing an axis should be done in the add_subplot command using sharex=first_axis_instance

I have put together an example which you should be able to run:

import matplotlib.pyplot as plt
import matplotlib.ticker as mticker
import matplotlib.dates as mdates


import datetime as dt


n_pts = 10
dates = [dt.datetime.now() + dt.timedelta(days=i) for i in range(n_pts)]

ax1 = plt.subplot(2, 1, 1)
ax1.plot(dates, range(10))

ax2 = plt.subplot(2, 1, 2, sharex=ax1)
ax2.bar(dates, range(10, 20))

# Now format the x axis. This *MUST* be done after all sharex commands are run.

# put no more than 10 ticks on the date axis.  
ax1.xaxis.set_major_locator(mticker.MaxNLocator(10))
# format the date in our own way.
ax1.xaxis.set_major_formatter(mdates.DateFormatter('%Y-%m-%d'))

# rotate the labels on both date axes
for label in ax1.xaxis.get_ticklabels():
    label.set_rotation(30)
for label in ax2.xaxis.get_ticklabels():
    label.set_rotation(30)

# tweak the subplot spacing to fit the rotated labels correctly
plt.subplots_adjust(hspace=0.35, bottom=0.125)

plt.show()

Hope that helps.

Answer 2

You should change this line:

ax = fig.add_subplot(111)

to

ax = fig.add_subplot(211)

The original command means that there is one row and one column so it occupies the whole graph. So your second graph fig.add_subplot(212) cover the lower part of the first graph.

Edit

If you dont want the gap between two plots, use subplots_adjust() to change the size of the subplots margin.

Answer 3

The example from @Pelson, simplified.

import matplotlib.pyplot as plt
import datetime as dt

#Two subplots that share one x axis
fig,ax=plt.subplots(2,sharex=True)

#plot data
n_pts = 10
dates = [dt.datetime.now() + dt.timedelta(days=i) for i in range(n_pts)]
ax[0].bar(dates, range(10, 20))
ax[1].plot(dates, range(10))

#rotate and format the dates on the x axis
fig.autofmt_xdate()

The subplots sharing an x-axis are created in one line, which is convenient when you want more than two subplots:

fig, ax = plt.subplots(number_of_subplots, sharex=True)

To format the date correctly on the x axis, we can simply use fig.autofmt_xdate()

For additional informations, see shared axis demo and date demo from the pylab examples. This example ran on Python3, matplotlib 1.5.1