I am trying to edit the d3_views module in this way:
adding 2 forms in the plugin d3_views_plugin_style_d3.inc:
1.form de type managed file: with this form I want to upload a photo to be used as a backgroud photo for the graph..

$form['Background_image'] = array(
    '#type' => 'managed_file',
    '#title' => t('Choose a background image'),
    '#description' => t('Click "Browse..." to select an image to upload.'),
     '#upload_location' => 'public://backgroundimage/',
     '#default_value' => $this->options['Background_image'],
    );

2.form de type select that display all the text files in the default directory of files: the file.text contains data in the format TVS file.
$form['Get_file'] = array(
         '#title' => 'Choose a project',
        '#type' => 'select',
        '#options' =>$this->get_projects(),
        '#default_value' => $this->options['Get_file'],
      );

function get_projects(){
  $directory= variable_get('file_directory_path', 'sites/default/files');
  $filenames = array();
  $extension =".txt";
  $iterator = new DirectoryIterator($directory);
  foreach ($iterator as $fileinfo) {
    if ($fileinfo->isFile()) {
        //check the extension ".txt"
        if (strpos($fileinfo->getFilename(),'.txt') !== false)
        {$filenames[$fileinfo->getMTime()] =substr($fileinfo->getFilename(), 0, -4);}
    }
   }
  return $filenames;
  }

The Idea is to get the image and the data file ,draw a graph with the data that exist in the data file and use the selected photo as a background photo of the final graph.

This function collect the required informations: data in order to return the graph

  function draw($rows) {
    $columns = $this->columns();
    // I added an element that should contains image obj and the data in the selected txt file
    //I definied a function that return the content of a file but still need how to create an image object from the selected image?
    $extradata=[];
    $legend = array();
    foreach ($columns as $column => $info) {
      $legend[] = $this->view->field[$column]->label();
    }
    $this->get_libraries();
    $this->get_projects();
    $type = $this->libraries[$this->options['library']]['js callback'];
    $chart = array(
      'id' => 'visualization',
      'type' => $type,
      'legend' => $legend,
      'rows' => $rows,
      'data' => $extradata,
    );
    return d3_draw($chart);
  }

//function how to read the content of a file : the problem is how to get the file path automaticly from the selected file in the form

<?php
function open_file($path)
{
   
// I just test the function with a concrete path but I need to get automaticaly the path of the selected file from the get_file form
   
$path= variable_get('file_directory_path', 'sites/default/files').'/file.txt';
  
$data = file($path);
   return
$data;
}
?>