Hi Olha,
It would be better if you posted the code you are using rather than an image. Assuming you are using ListPlot with a list of values, you can sample evenly spaced values from the list. e.g.
data = Range[10]
(* {1, 2, 3, 4, 5, 6, 7, 8, 9, 10} *)
data[[;; ;; 2]]
(* {1, 3, 5, 7, 9} *)
Then plot the sampled list.
