If stroke is specified, default span = I(1)

[cpsievert]

So that

plot_ly(x = letters[1:10], y = 1:10, stroke = I("black"))

gives

plot_ly(x = letters[1:10], y = 1:10, stroke = I("black"), span = I(1))

[cpsievert]

Done in 5b1f69c