Document graph measurement boundaries and improve label contrast

tools/chatgui/src/call_audio_graphs.cpp

@@ -41,7 +41,7 @@ void CallAudioGraphs::hideEvent(QHideEvent* event) {
 
 void CallAudioGraphs::paintEvent(QPaintEvent*) {
     QPainter p(this); p.fillRect(rect(), palette().window()); p.setRenderHint(QPainter::Antialiasing);
-    const QColor foreground = palette().text().color(), muted = palette().color(QPalette::Disabled, QPalette::Text);
+    const QColor foreground = palette().text().color(); QColor muted = foreground; muted.setAlphaF(.72);
     const QColor blue("#3498db"), green("#27ae60"), orange("#e67e22"), violet("#9b59b6"), red("#e74c3c");
     p.setPen(foreground); p.drawText(QRectF(16, 8, width() - 32, 22), text("Audio history — last 30 seconds"));
     p.setPen(muted);