Hide the scrollbar on the lobby graph

When hovering over a hook bordering the bottom of the lobby graph, a
scrollbar appears, probably because the icon grows in size to expand
below the box.

ui/lobby/css/app/_hook-chart.scss

@@ -2,6 +2,7 @@
   @extend %abs-100;
   bottom: 0;
   right: 0;
+  overflow: hidden;
   .label {
     color: $c-font-dim;
     font-size: .7em;