cabana: show dots when zoomed far into a signal (#27145)

* cabana: show dots when zoomed far into a signal

* review comments

tools/cabana/chartswidget.cc

@@ -469,6 +469,18 @@ void ChartView::updatePlot(double cur, double min, double max) {
     axis_x->setRange(min, max);
     updateAxisY();
   }
+
+  // Show points when zoomed in enough
+  for (auto &s : sigs) {
+    auto begin = std::lower_bound(s.vals.begin(), s.vals.end(), axis_x->min(), [](auto &p, double x) { return p.x() < x; });
+    auto end = std::lower_bound(s.vals.begin(), s.vals.end(), axis_x->max(), [](auto &p, double x) { return p.x() < x; });
+
+    int num_points = std::max<int>(end - begin, 1);
+    int pixels_per_point = width() / num_points;
+
+    s.series->setPointsVisible(pixels_per_point > 20);
+  }
+
   scene()->invalidate({}, QGraphicsScene::ForegroundLayer);
 }