Consider using ggplot2, see this example
library(ggplot2)
df <- data.frame(stringsAsFactors=FALSE,
Who = c("Ch", "Ch", "Ch", "Ch", "Ch", "Ch", "Ch", "Ch", "Ch",
"Ch"),
Activity = c("Stationary", "Stationary", "Interaction", "Stationary",
"Interaction", "Display", "Interaction", "Stationary",
"Stationary", "Stationary"),
Visitors = c(0, 20, 0, 30, 30, 30, 0, 0, 20, 30),
Duration = c(14, 18, 2, 6, 1, 10, 6, 5, 20, 13),
Visitor_Density = c("High", "High", "High", "High", "High", "High", "High",
"High", "High", "High")
)
ggplot(df, aes(x = Activity, y = Duration, color = Activity)) +
geom_boxplot()

Created on 2019-10-23 by the reprex package (v0.3.0.9000)