changes after I save a post

I would handle this with a shortcode myself. Something like this in your functions.php:

function wpa_85620_calculator_shortcode( $atts ) {
    extract( shortcode_atts( array(
        'key' => 'wr4nht4qbp',
        'width' => 800,
        'height' => 800
    ), $atts ) );

    return sprintf( '<object data="https://www.desmos.com/calculator/%s" width="%d" height="%d"></object>', $key, $width, $height );
}
add_shortcode( 'calculator', 'wpa_85620_calculator_shortcode' );

In your post/page:

[calculator] or [calculator width="400" height="200"]

As a plug-in (save as calc.php in your plugins folder):

<?php
/*
Plugin Name: Desmos Calculator
Plugin URI: http://wordpress.stackexchange.com/questions/85620/object-data-object-changes-after-i-save-a-post/85669#85669
Description: Desmos calculator shortcode
Version: 0.1
Author: Joseph Leedy
Author Email: [email protected]
License:

  Copyright 2013 Joseph Leedy ([email protected])

  This program is free software; you can redistribute it and/or modify
  it under the terms of the GNU General Public License, version 2, as 
  published by the Free Software Foundation.

  This program is distributed in the hope that it will be useful,
  but WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  GNU General Public License for more details.

  You should have received a copy of the GNU General Public License
  along with this program; if not, write to the Free Software
  Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
*/

function wpa_85620_calculator_shortcode( $atts ) {
    extract( shortcode_atts( array(
        'key' => 'wr4nht4qbp',
        'width' => 800,
        'height' => 800
    ), $atts ) );

    return sprintf( '<object data="https://www.desmos.com/calculator/%s" width="%d" height="%d"></object>', $key, $width, $height );
}
add_shortcode( 'calculator', 'wpa_85620_calculator_shortcode' );