Edge cases in eventplot are likely broken

[anntzer]

The eventplot code currently contains the snippet

        if len(lineoffsets) == 0:
            lineoffsets = [None]
        if len(linelengths) == 0:
            linelengths = [None]
        if len(linewidths) == 0:
            lineoffsets = [None] # oops
        if len(linewidths) == 0:
            lineoffsets = [None] # oops
        if len(colors) == 0:
            colors = [None]

which is certainly incorrect.

[dstansby]

For reference, some code that currently fails:

import matplotlib.pyplot as plt
plt.eventplot([1, 2, 3], lineoffsets=[], linelengths=[], linewidths=[], linestyles=[], colors=[])

[tacaswell]

It is not obvious that this is wrong, these values get passed into an EventCollection __init__ which may use it's default in place of the None (but I have not chased down the code path to see if it actually happens).

[oscargus]

I hijack this issue as this is also an edge case.

Is eventplot(1) supposed to work?

It is handled in the code:

matplotlib/lib/matplotlib/axes/_axes.py

Lines 1263 to 1264 in 1791022

	if not np.iterable(positions):
	positions = [positions]

But breaks later since the content is not a 1D-array.

Replacing the second line with positions = [np.asanyarray([positions])] makes it work, but maybe there should be an error already there?

The docs says:

matplotlib/lib/matplotlib/axes/_axes.py

Lines 1176 to 1177 in 1791022

	positions : array-like or list of array-like
	A 1D array-like defines the positions of one sequence of events.

Also, (related from the perspective that I am trying to increase test coverage) these two lines seems impossible to trigger:

matplotlib/lib/matplotlib/axes/_axes.py

Lines 1270 to 1271 in 1791022

	if len(positions) == 0:
	return []

[oscargus]

Considering the original issue, #7602 fixed the copy-paste-error, but was not considered a suitable solution.

What about just deleting those lines? Then there will be an error for empty (not the same length), which is fair enough. Or error already when detected that they are empty? (As they actually do not have to be the same length, just not empty.)

[timhoffm]

Is eventplot(1) supposed to work?

I'd say no. It's not documented to work, it does not work and nobody has complained that it should. Making it work would need additional code, documentation and tests. So let's not jump extra hoops here.

[github-actions]

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[QuLogic]

There is an open PR for this #22286; I guess a decision needs to be made there, so I added it to the agenda.