added diff column

Co-Authored-By: dantleech <530801+dantleech@users.noreply.github.com>

Author: staabm

phpbench.json

@@ -7,7 +7,12 @@
   "report.generators": {
     "my-report": {
       "generator": "expression",
-      "aggregate": ["iteration_index"],
+      "aggregate": [
+        "iteration_index"
+      ],
+      "expressions": {
+        "_mode": "mode(result_time_avg)"
+      },
       "cols": [
         "set",
         "mem_peak",
@@ -16,7 +21,7 @@
         "diff"
       ],
       "derivations": {
-        "diff": "if(min(table['mode']) > 0, format('%.2fx', row['mode'] / min(table['mode'])), 'N/A')"
+        "diff": "if(min(table['_mode']) > 0, format('%.2fx', row['_mode'] / min(table['_mode'])), 'N/A')"
       }
     }
   }