理解matplotlib绘图

import matplotlib.pyplot as plt

plt.plot([0, 1], [0, 1])      # plot a line from (0, 0) to (1, 1)
plt.title("a strait line")
plt.xlabel("x value")
plt.ylabel("y value")
plt.savefig("demo.jpg")
plt.show()

# 源码1
def plot(*args, **kwargs):
    ax = gca() #(1) 获得axes
    # allow callers to override the hold state by passing hold=True|False
    washold = ax.ishold()
    hold = kwargs.pop(‘hold‘, None)
    if hold is not None:
        ax.hold(hold)
    try:
        ret = ax.plot(*args, **kwargs) #(2) 调用ax.plot()
        draw_if_interactive()
    finally:
        ax.hold(washold)

    return ret

# 源码2
def gca(**kwargs):
    """
    Get the current :class:`~matplotlib.axes.Axes` instance on the
    current figure matching the given keyword args, or create one.

        If the current axes doesn‘t exist, or isn‘t a polar one, the appropriate
    axes will be created and then returned.

      """
    ax =  gcf().gca(**kwargs)
    return ax

#源码 3
def gcf():
    "Get a reference to the current figure."

    figManager = _pylab_helpers.Gcf.get_active() #(1) 从figure的active 列表中取出最后面的一个，如果列表为空，则返回None
    if figManager is not None:
        return figManager.canvas.figure #(2) 直接返回当前active figure列表中最上面的那个figure
    else:
        return figure()  #(3)  如果没有则创建一个

#源码4 figure类的gca
def gca(self, **kwargs):
        """
        Get the current axes, creating one if necessary

        The following kwargs are supported for ensuring the returned axes
        adheres to the given projection etc., and for axes creation if
        the active axes does not exist:

        %(Axes)s

        """
        ckey, cax = self._axstack.current_key_axes() #(1) 粗糙axes的是stack
        # if there exists an axes on the stack see if it maches
        # the desired axes configuration
        if cax is not None:

            # if no kwargs are given just return the current axes
            # this is a convenience for gca() on axes such as polar etc.
            if not kwargs:
                return cax

            # if the user has specified particular projection detail
            # then build up a key which can represent this
            else:
                # we don‘t want to modify the original kwargs
                # so take a copy so that we can do what we like to it
                kwargs_copy = kwargs.copy()
                projection_class, _, key = process_projection_requirements(
                    self, **kwargs_copy)

                # let the returned axes have any gridspec by removing it from
                # the key
                ckey = ckey[1:]
                key = key[1:]

                # if the cax matches this key then return the axes, otherwise
                # continue and a new axes will be created
                if key == ckey and isinstance(cax, projection_class):
                    return cax

        # no axes found, so create one which spans the figure
        return self.add_subplot(1, 1, 1, **kwargs)

fig1 = plt.figure(‘fig1‘) #获得一个figure, 这个figure是接下来所有画在这上面对象的载体。可以理解为边框
fig1_axes_1 = fig1.add_axes([0.1, 0.1, 0.8, 0.8])
fig1_axes_1.plot([0,1],[0,1])

# object-oriented plot

from matplotlib.figure import Figure
from matplotlib.backends.backend_agg import FigureCanvasAgg as FigureCanvas

fig    = Figure()
canvas = FigureCanvas(fig)
ax     = fig.add_axes([0.1, 0.1, 0.8, 0.8])

line,  = ax.plot([0,1], [0,1])
ax.set_title("a straight line (OO)")
ax.set_xlabel("x value")
ax.set_ylabel("y value")

canvas.print_figure(‘demo.jpg‘)

from matplotlib.figure import Figure
from matplotlib.backends.backend_agg import FigureCanvasAgg as FigureCanvas

fig    = Figure()
canvas = FigureCanvas(fig)

# first axes
ax1    = fig.add_axes([0.1, 0.1, 0.2, 0.2])
line,  = ax1.plot([0,1], [0,1])
ax1.set_title("ax1")

# second axes
ax2    = fig.add_axes([0.4, 0.3, 0.4, 0.5])
sca    = ax2.scatter([1,3,5],[2,1,2])
ax2.set_title("ax2")

canvas.print_figure(‘demo.jpg‘)