I want to replicate the right graph of this page :(

[parkjihun86 (Jihun Park)]

I don’t know how to express the function, and I doubt that it is a function because… when x=40, the y value seems to have two values… right?

Any ideas plz

[henrikschachner]

You probably need ParametricPlot:

functs = 50 {Cos[(x - 3)/4.4], Cos[(x - 7)/4.4]};
Plot[functs, {x, 0, 10}, GridLines -> {{3, 7}, None}]
Show[ParametricPlot[Evaluate@functs, {x, 0, 10}], 
 ParametricPlot[Evaluate@functs, {x, 3, 7}, 
  PlotStyle -> {Blue, Thickness[.01]}]]

[parkjihun86 (Jihun Park)]

Thank you so much! I should study about the function~!