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Improve zorder demo #15619

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85 changes: 44 additions & 41 deletions examples/misc/zorder_demo.py
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Zorder Demo
===========

The default drawing order for axes is patches, lines, text. This
order is determined by the zorder attribute. The following defaults
are set
The drawing order of artists is determined by their ``zorder`` attribute, which
is a floating point number. Artists with higher ``zorder`` are drawn on top.
You can change the order for individual artists by setting their ``zorder``.
The default value depends on the type of the Artist:

======================= =======
Artist Z-order
======================= =======
Patch / PatchCollection 1
Line2D / LineCollection 2
Text 3
======================= =======
================================================================ =======
Artist Z-order
================================================================ =======
Images (`.AxesImage`, `.FigureImage`, `.BboxImage`) 0
`.Patch`, `.PatchCollection` 1
`.Line2D`, `.LineCollection` (including minor ticks, grid lines) 2
Major ticks 2.01
`.Text` (including axes labels and titles) 3
`.Legend` 5
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Many of these classes are a bit obscure to users. Can we include the Axes calls that make them?

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I agree that these classes are a bit obscure. OTOH, the Z-order is exactly attached to these classes. It does not match too well to Axes calls, at least I found no consistent and descriptive way: Line2D can be created by a number of methods: plot (and all of its derivatives like semilogx, ...), axh/vline, scatter (depending on the parameters), ... Major ticks are implicitly made, there's no Axes method that create them. Even though Text and Rectangle are also building blocks for tables, I would not reference table() as a patch-creating method.

Therefore, I sticked to the existing structure of the table and just added more entries. Ideas to improve on that are always welcome.

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I would just do e.g. ax.plot, including minor... without trying to be comprehensive...

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Feel free to push an update if you like. I'm not feeling like fiddling what commands should get added right now.

Anyway, IMHO this PR stands for for itself and could get merged as an incremental improvement as is. Further improvements are always possible later.

================================================================ =======

You can change the order for individual artists by setting the zorder. Any
individual plot() call can set a value for the zorder of that particular item.
Any call to a plotting method can set a value for the zorder of that particular
item explicitly.

In the fist subplot below, the lines are drawn above the patch
collection from the scatter, which is the default.
.. note::

In the subplot below, the order is reversed.

The second figure shows how to control the zorder of individual lines.
`~.axes.Axes.set_axisbelow` and :rc:`axes.axisbelow` can further modify the
zorder of ticks and grid lines.
"""

import matplotlib.pyplot as plt
import numpy as np

# Fixing random state for reproducibility
np.random.seed(19680801)
r = np.linspace(0.3, 1, 30)
theta = np.linspace(0, 4*np.pi, 30)
x = r * np.sin(theta)
y = r * np.cos(theta)

###############################################################################
# The following example contains a `.Line2D` created by `~.axes.Axes.plot()`
# and the dots (a `.PatchCollection`) created by `~.axes.Axes.scatter()`.
# Hence, by default the dots are below the line (first subplot).
# In the second subplot, the ``zorder`` is set explicitly to move the dots
# on top of the line.

x = np.random.random(20)
y = np.random.random(20)
fig, (ax1, ax2) = plt.subplots(1, 2, figsize=(6, 3.2))

###############################################################################
# Lines on top of scatter
ax1.plot(x, y, 'C3', lw=3)
ax1.scatter(x, y, s=120)
ax1.set_title('Lines on top of dots')

plt.figure()
plt.subplot(211)
plt.plot(x, y, 'C3', lw=3)
plt.scatter(x, y, s=120)
plt.title('Lines on top of dots')
ax2.plot(x, y, 'C3', lw=3)
ax2.scatter(x, y, s=120, zorder=2.5) # move dots on top of line
ax2.set_title('Dots on top of lines')

# Scatter plot on top of lines
plt.subplot(212)
plt.plot(x, y, 'C3', zorder=1, lw=3)
plt.scatter(x, y, s=120, zorder=2)
plt.title('Dots on top of lines')
plt.tight_layout()

###############################################################################
# A new figure, with individually ordered items
# Many functions that create a visible object accepts a ``zorder`` parameter.
# Alternatively, you can call ``set_order()`` on the created object later.

x = np.linspace(0, 2*np.pi, 100)
plt.rcParams['lines.linewidth'] = 10
x = np.linspace(0, 7.5, 100)
plt.rcParams['lines.linewidth'] = 5
plt.figure()
plt.plot(x, np.sin(x), label='zorder=10', zorder=10) # on top
plt.plot(x, np.sin(1.1*x), label='zorder=1', zorder=1) # bottom
plt.plot(x, np.sin(1.2*x), label='zorder=3', zorder=3)
plt.axhline(0, label='zorder=2', color='grey', zorder=2)
plt.plot(x, np.sin(x), label='zorder=2', zorder=2) # bottom
plt.plot(x, np.sin(x+0.5), label='zorder=3', zorder=3)
plt.axhline(0, label='zorder=2.5', color='lightgrey', zorder=2.5)
plt.title('Custom order of elements')
l = plt.legend(loc='upper right')
l.set_zorder(20) # put the legend on top
l.set_zorder(2.5) # legend between blue and orange line
plt.show()