PreviewPerlin.cs

using UnityEngine;
using System.Collections;

public class PreviewPerlin : MonoBehaviour
{

    public int samples = 5000;
    public float previewLength = 10;

    public float xValue = 1;
    public float offset = 0;

    public float frequency = 1;
    public float gain = 1;

    void OnDrawGizmos()
    {
        samples = Mathf.Clamp(samples, 2, int.MaxValue);

        Vector3 lastPoint = Vector3.zero;

        float dist = previewLength / (float) samples;

        for (int i = 0; i < samples + 1; i++)
        {
            float height = (-0.5f + Mathf.PerlinNoise(xValue, offset + frequency * dist * i)) * gain;
            Vector3 thisPoint = new Vector3(dist * i, height);

            if (i != 0)
                Gizmos.DrawLine(lastPoint, thisPoint);

            lastPoint = thisPoint;
        }
    }
}