Render chart with data from Views PHP custom field
A work around for issue 1. &amp; 2. Add Bar number as a field to each study node. Then add field to view, and aggregate. 

How exactly is the Bar Number being calculated? You could use Rules to calculate this value when node is saved and save the calc number to your Bar number field. If there is no calculation, then ignore this part. 

Use field permissions to not allow certain roles to change the value of this field, if needed.



Add a new field called Bar Number (machine name: field_bar_number) to your study content type. 

You can create a custom module.

mymodule.info

name = My Module
description = Makes calculation for Bar Number field in Study content type.
package = Custom
version = 7.x-1.0
core = 7.x


mymodule.module

/**
 * Implements of hook_node_presave
 */
function MYMODULE_node_presave($node) {
  if ($node-&gt;type =='study') {
    // calculates bar number
    $node-&gt;field_bar_number = $node-&gt;some_field - $node-&gt;other_field;                          
  }
}


Use Views Bulk Operations and set title = title, and save again all your 500 nodes. As your custom module makes the calculation and sets the bar number before the node is saved. VBO Tutorial.