How can I have legends show in this linear graph?

[jlee0179]

Hello,

I am trying to create a linear graph with the code as following. However the output is not showing the legend, as shown in the plot after the code. My question is: How to include legend in the graph?

I'd appreciate for any helps!

ggplot(data = USArrests, aes(x = Murder)) +
  geom_line(aes(y = Assault), color = "steelblue") +
  geom_line(aes(y = UrbanPop), color = "darkred") +
  ylab("Count") 
#> Error in ggplot(data = USArrests, aes(x = Murder)): could not find function "ggplot"

^{Created on 2020-08-14 by the reprex package (v0.3.0)}

Session info

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