The d3 sankey libraries accept 0 as a possible number of iterations.

modules/d3_sankey_table_group_pp/d3_sankey_table_group_pp.module

@@ -109,7 +109,7 @@ function d3_sankey_table_group_pp_field_formatter_settings_form($field, $instanc
     $element['iterations'] = array(
       '#type' => 'textfield',
       '#title' => t('Iterations'),
-      '#element_validate' => array('element_validate_integer_positive'),
+      '#element_validate' => array('_d3_sankey_table_group_pp_element_validate_integer_positive_or_zero'),
       '#default_value' => $settings['iterations'],
     );
 
@@ -343,3 +343,15 @@ function _d3_sankey_table_group_pp_default_chart_options() {
     'type' => 'sankey',
   );
 }
+
+/* Form callbacks. */
+
+/**
+ * Form element validation handler for integers that must be positive or zero.
+ */
+function _d3_sankey_table_group_pp_element_validate_integer_positive_or_zero($element, &$form_state) {
+  $value = $element['#value'];
+  if ($value !== '' && (!is_numeric($value) || intval($value) != $value || $value < 0)) {
+    form_error($element, t('%name must be a positive integer or zero.', array('%name' => $element['#title'])));
+  }
+}