I started playing with the Drupal charts APIs a while ago, which uses Google's excellent charting APIs. This mostly worked well, except that the bar charts "class" did not support modifying the spacing between groups of bars. So, here's a small patch that solves this.

--- chart.module.orig   2008-12-23 09:53:48.000000000 -0800
+++ chart.module        2008-12-23 09:52:01.000000000 -0800
@@ -549,7 +549,7 @@
       
     // Bar chart bar sizing
     case 'chbh':
-      $data[$attr] .= implode(',', array($value['#size'], $value['#spacing']));
+      $data[$attr] .= implode(',', array($value['#size'], $value['#spacing'], $value['#group_spacing']));
       break;
       
      // Mixed axis positions, labels and styles
@@ -1028,10 +1028,11 @@
 * 
 * @return array
 */
-function chart_bar_size($size = 40, $spacing = 20) {
+function chart_bar_size($size = 40, $spacing = 20, $group_spacing = 30) {
   return array(       
       '#size'    => $size,  
       '#spacing' => $spacing,  
+      '#group_spacing' => $group_spacing,  
     );
 }