[Bug]: Possible bug on arrows in annotation

Bug summary

Arrow behavior in axes.annotate is inconsistent depending on the direction of the xy coordinate.

Arrow line begins at center of text in cases of horizontal/vertical lines, but begins at lower left of text in cases of diagonal lines.

Code for reproduction

import matplotlib.pyplot as plt

fig = plt.figure()
ax = fig.add_subplot(1,1,1)
ax.set_xlim(0,1)
ax.set_ylim(0,1)
ax.set_aspect('equal')
ax.grid(True)
for target in ((0.2,0.2),(0.2,0.4),(0.2,0.6),(0.4,0.2),(0.6,0.2)):
    ax.annotate('Blah! Blah blah blah.',xy=target, xytext=(0.6,0.6),
                 bbox=dict(boxstyle='round', facecolor='none'),
                 ha='center',va='center',
                 arrowprops=dict(facecolor='black',width=0.8,headwidth=5,connectionstyle="arc3,rad=0"))

Actual outcome:

Expected outcome

Arrow starting point should be consistent, or at least adjustable; relpos is not available as a FancyArrowPatch key, so if I add it, I get AttributeError: 'FancyArrowPatch' object has no property 'relpos'

Additional information

No response

Operating system

Windows

Matplotlib Version

import matplotlib; print(matplotlib.version)

Matplotlib Backend

module://matplotlib_inline.backend_inline

Python version

3.7.4

Jupyter version

6.1.4

Installation

conda

The question is: On which angles do we want to change the anchor point?

Currently the rules seem to be:

exactly horizontal lines attach to the side centers.
everything else goes to the nearest x-position from horizontal center and edges.

import matplotlib.pyplot as plt

fig, ax = plt.subplots()
ax.set(xlim =(0, 2), ylim=(0, 2))
ax.set_aspect('equal')
ax.grid(True)

phi = np.linspace(0, 2*np.pi, 145)
x = np.cos(phi) + 1
y = np.sin(phi) + 1

for xi, yi in zip(x, y):
    ax.annotate('Blah! Blah blah blah.',xy=(xi, yi), xytext=(1, 1),
                 bbox=dict(boxstyle='round', facecolor='none'),
                 ha='center',va='center',
                 arrowprops=dict(facecolor='black',width=0.8,headwidth=5,connectionstyle="arc3,rad=0"))

The relevant logic is at

matplotlib/lib/matplotlib/text.py

Lines 1932 to 1937 in 73304d2

    
           # pick the corner of the text bbox closest to annotated point. 
        
           xpos = [(bbox.x0, 0), ((bbox.x0 + bbox.x1) / 2, 0.5), (bbox.x1, 1)] 
        
           ypos = [(bbox.y0, 0), ((bbox.y0 + bbox.y1) / 2, 0.5), (bbox.y1, 1)] 
        
           x, relposx = min(xpos, key=lambda v: abs(v[0] - x1)) 
        
           y, relposy = min(ypos, key=lambda v: abs(v[0] - y1)) 
        
           self._arrow_relpos = (relposx, relposy)

I think.

I don't mind the existing implementation if it's documented -- it was a bit of a surprise to me.

self._arrow_relpos = (relposx, relposy)

...or allow the user to override this somehow.

Eventually, I gave up trying to do what I wanted (just have arrows emanating from the center of the block by trying to use relpos as I mentioned in #21732 ) and took an approach in StackOverflow and just created separate annotations for the arrows and for the text.

@jason-sachs

what I wanted (just have arrows emanating from the center of the block by trying to use relpos as I mentioned in #21732 )

@jason-sachs that's possible if you specify an arrowstyle, e.g. arrowprops=dict(arrowstyle='-|>', facecolor='black'). You can also use relpos here if you want.
There are two different draw routines with different behavior and I hope #21743 makes this more clear.

Yeah, I tried that, but using arrowstyle doesn't let me get the type of arrowhead I want.

Which type of head would you want?

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[Bug]: Possible bug on arrows in annotation #21733

Bug summary

Code for reproduction

Actual outcome:

Expected outcome

Additional information

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	# pick the corner of the text bbox closest to annotated point.
	xpos = [(bbox.x0, 0), ((bbox.x0 + bbox.x1) / 2, 0.5), (bbox.x1, 1)]
	ypos = [(bbox.y0, 0), ((bbox.y0 + bbox.y1) / 2, 0.5), (bbox.y1, 1)]
	x, relposx = min(xpos, key=lambda v: abs(v[0] - x1))
	y, relposy = min(ypos, key=lambda v: abs(v[0] - y1))
	self._arrow_relpos = (relposx, relposy)

[Bug]: Possible bug on arrows in annotation #21733

Description

Bug summary

Code for reproduction

Actual outcome:

Expected outcome

Additional information

Operating system

Matplotlib Version

Matplotlib Backend

Python version

Jupyter version

Installation

Activity

timhoffm commented on Nov 24, 2021

anntzer commented on Nov 24, 2021

jason-sachs commented on Nov 24, 2021

jason-sachs commented on Nov 24, 2021

timhoffm commented on Nov 24, 2021

jason-sachs commented on Nov 29, 2021

timhoffm commented on Nov 29, 2021

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