Plotting Astropy objects in Matplotlib

Plotting quantities

Quantity objects can be conveniently plotted using matplotlib. This feature needs to be explicitly turned on:

>>> from astropy.visualization import quantity_support
>>> quantity_support()  
<astropy.visualization.units.MplQuantityConverter ...>

Then Quantity objects can be passed to matplotlib plotting functions. The axis labels are automatically labeled with the unit of the quantity:

from astropy import units as u
from astropy.visualization import quantity_support
quantity_support()
from matplotlib import pyplot as plt
plt.figure(figsize=(5,3))
plt.plot([1, 2, 3] * u.m)

(png, svg, pdf)

Quantities are automatically converted to the first unit set on a particular axis, so in the following, the y-axis remains in m even though the second line is given in cm:

plt.plot([1, 2, 3] * u.cm)

(png, svg, pdf)

Plotting a quantity with an incompatible unit will raise an exception. For example, calling plt.plot([1, 2, 3] * u.kg) (mass unit) to overplot on the plot above that is displaying length units.

To make sure unit support is turned off afterward, you can use quantity_support with a with statement:

with quantity_support():
    plt.plot([1, 2, 3] * u.m)

Plotting times

Matplotlib natively provides a mechanism for plotting dates and times on one or both of the axes, as described in Date tick labels. To make use of this, you can use the plot_date attribute of Time to get values in the time system used by Matplotlib.

However, in many cases, you will probably want to have more control over the precise scale and format to use for the tick labels, in which case you can make use of the time_support function. This feature needs to be explicitly turned on:

>>> from astropy.visualization import time_support
>>> time_support()  
<astropy.visualization.units.MplTimeConverter ...>

Once this is enabled, Time objects can be passed to matplotlib plotting functions. The axis labels are then automatically labeled with times formatted using the Time class:

from matplotlib import pyplot as plt
from astropy.time import Time
from astropy.visualization import time_support

time_support()

plt.figure(figsize=(5,3))
plt.plot(Time([58000, 59000, 62000], format='mjd'), [1.2, 3.3, 2.3])

(png, svg, pdf)

By default, the format and scale used for the plots is taken from the first time that Matplotlib encounters for a particular Axes instance. The format and scale can also be explicitly controlled by passing arguments to time_support:

time_support(format='mjd', scale='tai')
plt.figure(figsize=(5,3))
plt.plot(Time([50000, 52000, 54000], format='mjd'), [1.2, 3.3, 2.3])

(png, svg, pdf)

To make sure support for plotting times is turned off afterward, you can use time_support as a context manager:

with time_support(format='mjd', scale='tai'):
    plt.figure(figsize=(5,3))
    plt.plot(Time([50000, 52000, 54000], format='mjd'))