Change the size of legend markers...

[jklymak]

Closed a PR that did this, which was probably over engineered. But the main idea is probably still useful.

#11127 is a bit different, it would create many legend entries for a single scatterplot. The idea here was to unify the size of different scatter plot's handles. Many options are shown in the stackoverflow question "setting a fixed size for points in legend". The easiest is just:

legend = plt.legend()
for legobj in legend.legendHandles:
    legobj.set_sizes([64])

My faviourite, which I just added to that thread as well would be to use the update function of the Handler.

def update(handle, orig):
    handle.update_from(orig)
    handle.set_sizes([64])

plt.legend(handler_map={PathCollection : HandlerPathCollection(update_func=update)})

Originally posted by @ImportanceOfBeingErnest in #4257 (comment)

[h-vetinari]

Saved my day. This really should work out of the box. Why do the legend markers change size at all...? I get the point where the only legend info is the size of the marker, but often it's colors/categories etc. that should be in the legend first and foremost.

[ImportanceOfBeingErnest]

Why do the legend markers change size at all...?

Usually you want a legend to show the marker in the same size as it appears in the plot. For a scatter with different sizes, there is no general way to determine which of the used sizes would be the best to show in the legend - this requires some user input.

But of course feel free to make a suggestion here. Given two scatter plots

sc1 = ax.scatter(..., s=[5,10,15])
sc2 = ax.scatter(..., s=[32,64,128])

which "automatic" size would you expect to see in a legend?

[h-vetinari]

which "automatic" size would you expect to see in a legend?

Just the normal fontsize of the legend? Or, maybe more accurately, a constant size so that the markers in the legend do not overlap -- something visually pleasing w.r.t. to the already fixed linespacing size (IOW, maybe slightly larger than the fontsize, but smaller than fontsize+linespacing).

[ImportanceOfBeingErnest]

Actually, I don't know why #4247 wasn't pursued further. It looks much more straight forward than #4257.
I think @h-vetinari 's suggestion could be easily incoorporated as an uniform_size="auto" parameter (although it might not be possible to easily update the legend once fontsize or linespacing change after legend creation).

[bf]

It'd be great if this is further pursued by someone who is able to implement it. It is one of the most important things that's missing in my day-to-day pyplot use.

[github-actions]

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[h-vetinari]

Pity that this got closed, I missed the ping a year ago... 😑

[rcomer]

@h-vetinari we are happy to re-open these bot-closed issues if someone is still interested in them.