Website: fix gaps in chart

If you place the cursor on a mark and between chart rows, no row is
selected. This can cause flickering effects (see commit
37366aa).

Fix selection by making the row have mouse hit testing priority over
marks.

Author: strager

website/public/benchmarks/benchmark.css

@@ -273,20 +273,23 @@ body.javascript .javascript-only {
 .chart .tick .marker {
   z-index: 1;
 }
+.chart .series {
+  z-index: 2; /* For mouse events, not visuals. */
+}
 .chart .series .bar {
-  z-index: 2;
+  z-index: 3;
 }
 .chart .series .error-bars {
-  z-index: 3;
+  z-index: 4;
 }
 .chart .tick .label {
-  z-index: 4;
+  z-index: 5;
 }
 .chart .series .label {
-  z-index: 5;
+  z-index: 6;
 }
 .chart .log-scale {
-  z-index: 6;
+  z-index: 7;
 }
 
 .benchmark-table {