How do you force points into a straight a line using swipe?

So I'm working on a mobile game. One of the features requires you to swipe and create a straight a line. I did this one way with only two points, but I didn't like it because the line would only appear after your finger was released from the screen and you couldn't make the same line longer w/o drawing a whole new line. So I started re-coding it with the intention of having it work with multiple points.

Right now my code work as a tracer, it captures points when a finger is dragged across the screen. How do I force those points to be in a line?

Here is my code so far.

 using UnityEngine;
 using System.Collections;
 using System.Collections.Generic;    
 
 [RequireComponent (typeof (LineRenderer))]
 
 public class DrawLine : MonoBehaviour 
 {
 
     public float lineWidth = 0.1f;
     public float slopeMultiplier = 10.0f;
     public int sameLineCounter = 0;
     public int a = 1;
 
     public Vector2 lineOrigin;
     public Vector2 lineSlope;
     public Vector2 lastPoint;
 
     public Vector3 currentTouch;
 
     public List<Vector2> tempPoints;
     public List<Vector2> myPoints = new List<Vector2>();
     public List<Vector2> previousPoints = new List<Vector2>();
 
     public GameObject boxPrefab;
     public Transform boxClone;
     public LineRenderer lineRenderer;
 
     public MoveCollider moveColliderScript;
     
     void Start ()
     {
         lineRenderer = gameObject.GetComponent<LineRenderer>();
         lineRenderer.SetWidth(lineWidth,lineWidth);
     }
 
     void Update ()
     {
         if(Input.touchCount > 0)    //Checks to see if player begins touching the screen
         {
             switch(Input.GetTouch(0).phase)
             {
                 case TouchPhase.Began:
                     currentTouch = Input.GetTouch(0).position;
                     lineOrigin = currentTouch;                            //Sets the origin of the line to the first touch
                     previousPoints.Add(lineOrigin);
                     if(myPoints == null)
                     {
                         myPoints = new List<Vector2>();
                         Vector3 pos = lineOrigin;
                         pos.z = 8;
                         Vector3 lineOriginScreenPos = Camera.main.ScreenToWorldPoint(pos);
                         myPoints.Add(lineOriginScreenPos);
                         boxClone = Instantiate(boxPrefab, myPoints[0], Quaternion.identity) as Transform;    //Test to see where points are by creating a cube
                     }
                     break;
 
             case TouchPhase.Stationary:
                     break;
 
                 case TouchPhase.Moved:
                     sameLineCounter++;    
                     lineSlope = Input.GetTouch(0).position - lineOrigin;
                     AddPointInLine();            //Add points based on slope and previous point
                     //Debug.Log("Fired " + sameLineCounter + " time(s) on the same line.");        //Test to see how much times TouchPhase.Moved goes off
 
                     for(; a <= 1; a++)
                     {
                         lineSlope = Input.GetTouch(0).position - lineOrigin;                        //Captures the second touch
                         lineSlope = lineSlope * slopeMultiplier;
                         Debug.Log("Fired " + a + " times.");
                     }
                     
                     break;
 
                 case TouchPhase.Ended:
                     ResetLine();
                     break;
 
                 case TouchPhase.Canceled:
                     ResetLine();
                     break;
             }
             
         }
         else
         {
             myPoints = null;
             moveColliderScript.DestroyCollider();
         }
 
     }
     
     void AddPointInLine()
     {
         if(myPoints == null)
         {
             myPoints = new List<Vector2>();
             tempPoints = new List<Vector2>();
         }
         else
         {
             tempPoints = new List <Vector2>(myPoints.Count);
             for(int j = 0; j < myPoints.Count - 1; j++)
             {
                 tempPoints[j] = myPoints[j];
             }
         }
 
         if(tempPoints.Count == 0)    //Checks to see if list is empty to avoid setting last point to -1
         {
             lastPoint = lineOrigin;    //If it is sets lastPoint to the first point(lineOrigin)
         }
 
         /*if(tempPoints.Count > 0)
         {
             lastPoint = myPoints[myPoints.Count - 1];    //Otherwise lastPoint is the newest object in the list
         }*/
 
         Vector2 newPoint = lastPoint + lineSlope; //New point adds the last point and the slope to generate a point that is in a straight line
         previousPoints.Add(lastPoint);
         lastPoint = newPoint; //Gets the new generated point.
         Vector3 pos = newPoint;    //Converts newPoint to Vector3
         pos.z = 8;//Sets an arbitrary number for depth
         Vector3 newPointScreenPos = Camera.main.ScreenToWorldPoint(pos);//Converts newPoint to screen space
         tempPoints.Add(newPointScreenPos);    //Adds it to the list
 
         myPoints = new List <Vector2>(tempPoints.Count);
         myPoints = tempPoints;
 
         boxClone = Instantiate(boxPrefab, myPoints[myPoints.Count - 1], Quaternion.identity) as Transform; //Test to see where points are by creating a cube
     }
 
     void ResetLine()    //Resets the vars so the next line swipe will start fresh
     {
         lineOrigin = new Vector2(0,0);    
         lineSlope = new Vector2(0,0);
         myPoints = null;
         sameLineCounter = 0;
     }
     
 }