Feature request: Add argument "fill" to lineplot()

kdeplot() offers an argument fill. It would be nice to offer this argument to lineplot() as well.

sns.lineplot(
   data=data, x="x", y="y", hue="category", fill=True, palette="crest", alpha=.5, linewidth=0
)

(image from kdeplot docu)

Keep in mind that the lineplot can be used in the context of a polar plot (axis projection: “polar”).

PS: happy New Year!

[mwaskom]

But isn’t seaborn’s theme, hue/style selection or handling of the legend superior compared to pandas’ plotting features?

Not really, because seaborn does all of its theming through the matplotlib rc system, which pandas plots also pick up. So if you set a theme “in seaborn”, it will affect pandas plots too.

I don’t think it makes much sense to have a lineplot that draws areas … lineplot uses filling to represent uncertainty which is different (in tension with, actually) using it to represent the quantity.

But … on the user side it’s easy to define a simple function that adds fills to a line plot, e.g. something like

def fill_under_lines(ax=None, alpha=.2, **kwargs):
    if ax is None:
        ax = plt.gca()
    for line in ax.lines:
        x, y = line.get_xydata().T
        ax.fill_between(x, 0, y, color=line.get_color(), alpha=alpha, **kwargs)

If you have that sitting in a personal function library, you can call it after you make a lineplot and it will add fills that match the lines in your plot, essentially just as you’d like.

[mwaskom]

I am -1 on adding this to lineplot, but am somewhat open to the idea of an areaplot. There are a few things one would need to think about though:

• There would need to be a way to show errorbars, I’m not really sure that there’s a good way to do that with area plots
• Currently, the way things work in seaborn is that all of the plots in the same module (and same figure-level interface) support the same semantic mappings. lineplot and scatterplot support hue, size, and style. It’s not obvious how to add size and style. Well, with style, you could do hatching, but I have been reluctant to add hatching to seaborn because of limitations in the matplotlib API for hatches. Perhaps it would be possible to relax the constraint, but it’s the kind of thing that would add some complexity and so would need a good motivation.
• Naturally the next step after giving the mouse the areaplot cookie is that they would want some “stacked” area plot milk. The logic that does stacking for filled densities or histograms assumes a different internal representation of the data than is in the relational plots. On a medium-long term time horizon, I’d like to make that more general, but I’d consider that a prerequisite to adding this.

So if the question is “should seaborn have an area plot that can’t do aggregation/errorbars and only has hue semantic mapping” my answer would probably be no because that exists in pandas builtin plotting and I try not to devote effort to straight up duplication of plots you can already make with matplotlib or pandas. If some of these problems can be solved, I’d consider it worth adding to seaborn. But it’s a low priority right now.