Bug report

I was making a stack plot and wanted the edge colors of the plotted lines to be the same as the fill color instead of the default white, so I set edgecolor='face'. This produces a value error when using plt.legend(). Without the legend function, there are no errors and it plots as expected. I tried changing different parameters of the legend() function, but didn't have any success.

Code for reproduction

importnumpyasnpimportpandasaspdimportmatplotlib.pyplotasplt# Create a datasetmy_count= ["France","Australia","Japan","USA","Germany","Congo","China","England","Spain","Greece","Marocco","South Africa","Indonesia","Peru","Chili","Brazil"]df=pd.DataFrame(    {"country":np.repeat(my_count,10),"years":list(range(2000,2010))*16,"value":np.random.rand(160)    })pivot=df.groupby(["country","years"])["value"].sum().unstack(0).cumsum()# Plot datasetplt.stackplot(pivot.index.values,pivot.T.values,labels=pivot.columns,alpha=0.2,edgecolors="face")plt.legend()plt.show()

Actual outcome

with the following value error:

ValueError: Using a string of single character colors as a color sequence is not supported. The colors can be passed as an explicit list instead.

Expected outcome

The same graph as above, but with a legend.

Matplotlib version

• Operating system: Microsoft Windows 10
• Matplotlib version: 3.4.2
• Matplotlib backend: module://ipykernel.pylab.backend_inline
• Python version: 3.8.5
• Other libraries: numpy, pandas (to create dataset)

Installed matplotlib using 'pip install matplotlib'.