Color in stripchart

Dear Robin and Gerrit,

I am unable to see the difference in the 
plot in the two cases ... 

	df <- data.frame(year = seq(2011, 2018), value = seq(10, 80, 10))
	df$color <- 'black'
	df[df$value<33,]$color <- 'blue'
	df[df$value>66,]$color <- 'red'
	
	file <- "/tmp/robin.png"
	width <- 1000
	height <- 1000
	res <- 150
	png(file=file, width=width, height=height, res=res)
	par(mfrow=c(2,1))
	
	stripchart(df$value ~ df$year, pch=18, col=df$color, method='stack', main="stripchart with color column as col")
	
	stripchart(df$value ~ df$year, pch=18, method='stack', main="stripchart with black basis")
	points(df[df$color=='blue',]$value, df[df$color=='blue',]$year-2010,
	type='p', pch=18, col='blue')
	points(df[df$color=='red',]$value, df[df$color=='red',]$year-2010,
	type='p', pch=18, col='red')
	
	dev.off()

Best,
Rasmus