Correct implementation of computeCartesianPath?

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Hello everyone. I'm quite new to ROS, doing a practicum project right now and have been pretty stumped with this for a few days. I have a TIAGO Robot, with a 6DOF arm with whom I want to be able to move on a straight line. The "straight" part is important as we'll be having a pressure sensor on the fingertip with which we want to do "feeling" of surfaces, so we need to be able to move it on a straight line (maintaining orientation of it) on the XY plane.

So I've been basically doing some frankencode because while I'm familiar with C++, I'm not with ROS and MoveIt!; and been piecing something together from a basic code in the TIAGO tutorials that moves the actuator to a cartesian point and orientation (but not necessarily in a straight line) and the moveit tutorial that deals with Cartesian path. Current code:

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/** \author Jordi Pages. */

// ROS headers
#include <ros/ros.h>
#include "shape_msgs/SolidPrimitive.h"
#include "moveit_msgs/BoundingVolume.h"


// MoveIt! headers
#include <moveit/move_group_interface/move_group_interface.h>
//#include <moveit_visual_tools/moveit_visual_tools.h>

// Std C++ headers
#include <string>
#include <vector>
#include <map>
#include <cmath>

int main(int argc, char** argv)
{
  ros::init(argc, argv, "plan_arm_torso_ik_idril");

  if ( argc < 7 )
  {
    ROS_INFO(" ");
    ROS_INFO("\tUsage:");
    ROS_INFO(" ");
    ROS_INFO("\trosrun tiago_moveit_tutorial plan_arm_torso_ik_idril  x y z  r p y");
    ROS_INFO(" ");
    ROS_INFO("\twhere the list of arguments specify the target pose of /arm_tool_link expressed in /base_footprint");
    ROS_INFO(" ");
    return EXIT_FAILURE;
  }

  geometry_msgs::PoseStamped goal_pose;
  goal_pose.header.frame_id = "base_footprint";
  goal_pose.pose.position.x = atof(argv[1]);
  goal_pose.pose.position.y = atof(argv[2]);
  goal_pose.pose.position.z = atof(argv ...