lmplot with refline produces unexpected markers on both ends of the lines

[hchau630]

The following simple example which calls lmplot and then calls FacetGrid.refline produces some unexpected gray markers on both ends of the lines plotted by refline. This behavior seems specific to lmplot since those markers are not present when I use it with other plot types. This is on version 0.13.2

import seaborn as sns
import pandas as pd
import matplotlib.pyplot as plt

df = pd.DataFrame({"x": [1.0, 2.0, 3.0], "y": [0.0, 1.0, 2.0]})
g = sns.lmplot(df, x="x", y="y")
g.refline(x=0, y=0)
plt.show()

[adityaanikam]

Traced this down to the exact mechanism, and it's less lmplot specific than it looks. lmplot always sets facets.hue_kws = {"marker": ["o"]} on the FacetGrid it returns, even with no hue mapping at all, markers="o" is the default. FacetGrid.map() then unconditionally injects every hue_kws entry into whatever function you hand it, so when refline calls self.map(plt.axvline, ...), marker="o" rides along into a function that was never meant to receive it. axvline returns a Line2D, which does accept marker and draws it at both vertices of the line, hence the gray dots on both ends. I'd like to fix refline to strip hue_kws before calling map, or map to skip injecting hue_kws for plain matplotlib functions.