I tried adding a simple arrow through PatchCollection without success.
If I'm using PatchCollection right it could explain why Axes.add_patches is necessary for streamplot() to show arrows...

I'm also confused about the use of transform in that particular case. In streamplot(), transform argument is specified at FancyArrowPatch creation and at PatchCollection creation.
Is it the right way of doing it ? because even a classic patch (the ellipse in the linked script) is not displayed right in that case.

import matplotlib.pyplot as plt
from matplotlib import patches, lines
from matplotlib import collections


def draw_test(ax, trans1, trans2, title):
    if trans1:
        transform1 = ax.transData
    else:
        transform1 = None
    if trans2:
        transform2 = ax.transData
    else:
        transform2 = None
    # Add arrow
    arrow_tail = [0, 0]
    arrow_head = [1, 0]
    p = patches.FancyArrowPatch(arrow_tail, arrow_head, arrowstyle='-|>',
                                mutation_scale=40, transform=transform1,
                                margins=False)
    ac = collections.PatchCollection([p], margins=False, transform=transform2)
    ax.add_collection(ac)
    # Add another kind of patch
    e = patches.Ellipse([0, 0], .2, .5, .5, transform=transform1)
    ec = collections.PatchCollection([e], margins=False, transform=transform2)
    ax.add_collection(ec)
    # Add lines
    streamline = [[0, -0.5], [0.5, 0], [1, 0.5]]
    lc = collections.LineCollection([streamline], margins=False,
                                     transform=transform2)
    ax.add_collection(lc)
    # 
    ax.set_title(title)
    ax.set_xlim(-1, 1)
    ax.set_ylim(-1, 1)


# Create subplot
fig, axs = plt.subplots(3, 2, figsize=(8, 8))
axs[0, 1].remove()

# Add as patches
ax1 = axs[0, 0]
transform1 = ax1.transData
arrow_tail = [0, 0]
arrow_head = [1, 0]
p1 = patches.FancyArrowPatch(arrow_tail, arrow_head, arrowstyle='-|>',
                             mutation_scale=40, transform=transform1,
                             margins=False)
ax1.add_patch(p1)
e1 = patches.Ellipse([0, 0], .2, .5, .5)
ax1.add_patch(e1)
streamline1 = [[0, -0.5], [0.5, 0], [1, 0.5]]
ax1.add_line(lines.Line2D(*zip(*streamline1)))
ax1.set_title("Added as patches")
ax1.set_xlim(-1, 1)
ax1.set_ylim(-1, 1)

# Add with various transform
draw_test(axs[1, 0], trans1=False, trans2=False, title="No transform used")
draw_test(axs[1, 1], trans1=True, trans2=False, title="Transform on patches")
draw_test(axs[2, 0], trans1=False, trans2=True, title="Transform on PatchCollection")
draw_test(axs[2, 1], trans1=True, trans2=True, title="Transform on both")

plt.tight_layout()
plt.show()

Uh oh!

Non-working 'set_visible' method for PatchCollection #6000

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