web: make loss and retransmission charts obvious, draw sparse series as points and keep percent axes within 100% [CI]

web/shared/theme/base.css

@@ -1088,3 +1088,20 @@ pre.code {
   align-items: center;
   gap: 4px;
 }
+
+/* 「没有才是好」的图整段为 0 时：盖一句说明，图的坐标轴留着，说明在绘图区中间 */
+.chart-body {
+  position: relative;
+}
+
+.chart-quiet {
+  position: absolute;
+  inset: 0 0 28px 56px;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  color: var(--app-ok);
+  font-size: 14px;
+  font-weight: 600;
+  pointer-events: none;
+}