Graph: set max slices in Categories pie chart

When there is a large number of Categories defined in the system, the
By Category pie chart is cramped and difficult to read.

This sets a (hardcoded for now) limit on the number of slices, and
regroups all the remaining categories under an "others" slice.

Fixes #25522

plugins/MantisGraph/lang/strings_english.txt

@@ -88,3 +88,4 @@ $s_plugin_MantisGraph_graph_page = 'Graph Issue History';
 $s_plugin_MantisGraph_graph_topdev = 'Top Developers by Fixed Issues';
 $s_plugin_MantisGraph_graph_opendev = 'Developers by Open Issues';
 $s_plugin_MantisGraph_graph_topreporter_fixed = 'Top Reporters by Fixed Issues';
+$s_plugin_MantisGraph_other_categories = 'other categories (%d)';

plugins/MantisGraph/pages/category_graph.php

@@ -38,6 +38,28 @@
 
 # Dynamically set width ratio between 1 and 0.25 based on number of categories
 $t_wfactor = 1 - min( max( count( $t_metrics ), 25 ) - 25, 75 ) / 100;
+
+# Set the maximum number of pie slices displayed and aggregate the rest into
+# an "others" category if needed. The number of slices should not be higher
+# than the number of available colors in the palette.
+$t_num_slices = 20;
+$t_pie_metrics = array_slice( $t_metrics, 0, $t_num_slices );
+if( count( $t_metrics ) > $t_num_slices ) {
+	$t_num_slices--;
+
+    # Remove last element and replace it with "others"
+    array_pop( $t_pie_metrics );
+	$t_others = sprintf(
+		plugin_lang_get( 'other_categories' ),
+		count( $t_metrics ) - $t_num_slices
+	);
+	$t_pie_metrics[$t_others] = 0;
+
+	# Sum remaining categories into "others" slice
+	foreach( array_slice( $t_metrics, $t_num_slices ) as $t_value ) {
+		$t_pie_metrics[$t_others] += $t_value;
+	}
+}
 ?>
 
 <div class="col-md-12 col-xs-12">
@@ -56,7 +78,7 @@
 		</div>
 
 		<div class="col-md-6 col-xs-12">
-			<?php graph_pie( $t_metrics ); ?>
+			<?php graph_pie( $t_pie_metrics ); ?>
 		</div>
 	</div>
 </div>