Can you please provide python code that generate the issue? As the julia folks point at us, it is not clear that this is not a bug on the julia side.

I tried just now. I'm afraid I'm too much of a beginner with both Julia and Matplotlib to figure this out. However, poster jheinen on the Julia forum claims to have reproduced this problem in pure Python. I suggest asking him for his code. I'm sorry I can't be of any more help. I don't want to get in the middle of any finger pointing between these two communities.

It is not finger pointing, just sorting out what the actual bug is.

attn @jheinen

This is how you can reproduce the bug (in Python)

import matplotlib.pyplot as plt
import numpy as np
plt.stem(np.random.randn(10000))

Fails on macOS X, can't check other backends before Monday ...

Indeed, this is likely due to the fact that stem generates one Line2D per stem, rather than grouping everything in a LineCollection, which would probably improve the performance quite a bit.

(but LineCollection currently cannot take a custom marker :-()

At a minimum, we could save some time by not going through plot

Having a look at this and think I'm making progress

I also confirmed this bug in Python 3 on Ubuntu 16.04. I tried to plot a stem plot with 130,000 points and the computer crunched on it a long time without displaying anything.

Since I encountered this problem by plotting around 30000 data points today I found this issue.
I can only link to this workaround I found, which solved my problem through manually calculating the line, coordinates and plotting a line collection, instead of using stem itself.
https://stackoverflow.com/questions/48301044/optimization-of-matplotlib-stem-plot
Maybe it helps someone.

Will this be merged to some 3.0.x version, or do we need to wait for stable 3.1.x version if we want this fixed in pip?

I'm afraid this is only going to be in 3.1.x onwards.

But 3.1.0 should hopefully come out soon 😄 .

Hopefully today or tomorrow.