[FIXED] basic questions about fig and axes in matplotlib

Issue

I am self-learning python, and I am very confused by the matplotlib package.

1. when plotting, is it really necessary to use the below code?

   fig = plt.figure()
   

I thought the code was to define an empty canvas, then we can add subplots to the canvas, but I also see lots of cases where this code is not included, and plt.plot() or plt.subplot() is directly used.

2. what are the differences between ax and axes? I used to think both were subplot variables, but later I find that axes is sometimes used as variables and sometimes as arguments in plotting, for instance:

as variables, from this post:

fig, axes = plt.subplots(nrows=3, ncols=2, figsize=(12, 8))

as an argument, from this post

n = len(fig.axes)

Any help would be much appreciated.

Solution

It is possible to plot without setting any variables. Example:

plt.figure()
plt.plot([1, 2], [5, 8])
plt.show()

You must initialize your figure somewhere, hence plt.figure(). As you pointed out, you can also use plt.subplots():

fig, ax = plt.subplots()
ax.plot([1, 2], [5, 8])
plt.show()

Note that we did not set the ncols and nrows keyword arguments. As the default value for both is 1, we get a single axis (which is why I chose the variable name ax). For n × 1 or 1 × n subplots, the second variable returned by plt.subplots() is a one-dimensional array.

fig, axes = plt.subplots(ncols=1, nrows=2)
axes[0].plot([1, 2], [5, 8])
axes[1].plot([2, 3], [9, 3])

In the case of m × n subplots (m, n > 1), axes is a two-dimensional array.

fig, axes = plt.subplots(ncols=2, nrows=2)
axes[0][0].plot([1, 2], [5, 8])
axes[1][0].plot([2, 3], [9, 3])
axes[0][1].plot([3, 4], [5, 8])
axes[1][1].plot([4, 5], [9, 3])

Whether you use ax or axes as the name for the second variable is your own choice, but axes suggests that there are multiple axes and ax that there is only one. Of course, there are also other ways to construct subplots, as shown in your linked post.

Answered By - tim