Visualizing classified objects/clusters of laserscan in rviz

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Line List is to draw lines for you case. Push start and end point of each line to line list variable in order.

It will draw a line between each pair of points, so 0-1, 2-3, 4-5, ...

This puslihing rviz marker lines tutorial is helpful.

    //some code with comments

    //publisher for line list marker
    ros::Publisher marker_pub = n.advertise<visualization_msgs::Marker>("visualization_marker", 10);

   //variable for line list marker    
    visualization_msgs::Marker line_list;

   // frame in which you have cluster coordinates
    line_list.header.frame_id = "/world_frame";  

   //marker type , scale,colour
    line_list.type = visualization_msgs::Marker::LINE_LIST;

   line_list.scale.x = 0.1;

    line_list.color.r = 1.0;
    line_list.color.a = 1.0;

    // assume you have 5 lines 
    for (int  i = 0; i < 5; ++i) 
        {
          geometry_msgs::Point line_start,line_end;

         // fill start and end coordinates 
          line_start.x = sx[i];
          line_start.y = sy[i];
          line_start.z = sz[i];

          line_end.x = ex[i];
          line_end.y = ey[i];
          line_end.z = ez[i];

          // The line list needs two points for each line
          line_list.points.push_back(line_start);
          line_list.points.push_back(line_end);
        }

        // publish marker
        marker_pub.publish(line_list);