using System.Linq;

  ...

  int[] y = File
    .ReadAllText(@"F:/C#/graph/graph/bin/Debug/y.txt")
    .Split(new char[] { ' ' }, StringSplitOptions.RemoveEmptyEntries)
    .Select(item => int.Parse(item))
    .ToArray();

  string[] x = File
    .ReadAllText(@"F:/C#/graph/graph/bin/Debug/x.txt")
    .Split(new char[] { ' ' }, StringSplitOptions.RemoveEmptyEntries)
    .ToArray();