R: CO2 Level Over Millenia Project: hline() label

Hi All,
I am working on CO2 Level Over Millenia Project. At the last step (18), there is requirement to put a label to the horizontal line which represents the Historical CO2 Peak before 1950. The hint is Assign the value of the linetype argument as "Historical CO2 Peak before 1950"
However, my understanding of linetype is the visual type of the line, for example, solid, dashed, etc.
Which argument should I use to label the line?

The linetype argument specifies the style of the line, for example, solid, dotted, or dashed. I believe what the authors of the project intended was for the user to add a label to the line. This can be achieved using an annotation layer. For example, adding the following code to your plot

+ geom_hline(yintercept = max(noaa_data$CO2_ppmv), linetype = "dashed") + annotate("text", x = 500, y = 303, label = "Historical CO2 Peak before 1950")

produces the following output:

The first argument specifies what kind of annotation to add (in this case, text), the x and y arguments specify the position of the annotation on the graph, and the label argument specifies the text to display.

For more information about annotation layers, see Create an annotation layer — annotate • ggplot2.