fix(plot,render,glfw): a hue-neutral crosshair, an em-dash placeholder, one layout per figure event, and a usable readout switch

src/plot/plot.cpp

@@ -1305,9 +1305,10 @@ void Plot::build_crosshair() {
     row.label_box = Rect{};
     row.value_box = Rect{};
     row.label.assign(trace.name.empty() ? crosshair_unnamed_label(i) : trace.name);
-    row.value.clear();
-    // The value, and with it the marker: a trace with no sample at this x writes no number
-    // and gets no marker, rather than a value clamped to the end of its series.
+    // The value: the interpolated number where the trace has a sample at this x, and the em dash
+    // where it does not. The row is listed either way, so the absence has to be *said* -- a blank
+    // value cell reads as a value that has not arrived -- and the dash is measured into the panel's
+    // width like any other value, so the row's boxes are placed by one rule.
     if (crosshair_value(i, crosshair_.x, row.y)) {
       row.valid = true;
       char value[32];
@@ -1319,6 +1320,8 @@ void Plot::build_crosshair() {
       // actually drawn however its axes are scaled.
       row.marker = Vec2{static_cast<f32>(x_transform(trace.axis_x).to_screen_x(crosshair_.x)),
                         static_cast<f32>(y_transform(trace.axis_y).to_screen_y(row.y))};
+    } else {
+      row.value.assign(kCrosshairNoValue);
     }
   }
   crosshair.rows.resize(count);
@@ -1328,13 +1331,13 @@ void Plot::build_crosshair() {
   if (count == 0u || !crosshair_.show_readout || text_ == nullptr) return;
 
   const ReadoutChrome chrome = readout_chrome(legend_, style_, *text_);
-  // Each row's content: its label, the gutter, and its value when it has one -- an invalid
-  // row is narrower, and the panel is sized by the widest row that will be listed, which the
-  // prefix-max reads in O(1) after a row was dropped from the end.
+  // Each row's content: its label, the gutter, and its value -- which is a number or the em dash,
+  // so every row is the same two runs and the panel is sized by the widest *listed* one, read in
+  // O(1) by the prefix-max after a row was dropped from the end.
   crosshair_content_px_.clear();
   for (const CrosshairRow& row : crosshair.rows) {
-    f32 content = text_->measure(row.label.c_str());
-    if (row.valid) content += chrome.pad + text_->measure(row.value.c_str());
+    const f32 content =
+        text_->measure(row.label.c_str()) + chrome.pad + text_->measure(row.value.c_str());
     crosshair_content_px_.push_back(crosshair_content_px_.empty()
                                         ? content
                                         : std::max(crosshair_content_px_.back(), content));
@@ -1393,11 +1396,12 @@ void Plot::build_crosshair() {
     row.swatch = readout_swatch(chrome, crosshair.panel, i);
     const Vec2 pen = readout_text_pen(chrome, crosshair.panel, i);
     row.label_box = Rect{pen.x, pen.y, text_->measure(row.label.c_str()), chrome.line_h};
-    if (row.valid) {
-      const f32 width = text_->measure(row.value.c_str());
-      row.value_box =
-          Rect{crosshair.panel.right() - chrome.pad - width, pen.y, width, chrome.line_h};
-    }
+    // Every listed row stages a value run: the number, or `kCrosshairNoValue` where the trace has
+    // no sample here. A blank cell would read as a value still to come, which is a different thing
+    // from "nothing at this x".
+    const f32 width = text_->measure(row.value.c_str());
+    row.value_box =
+        Rect{crosshair.panel.right() - chrome.pad - width, pen.y, width, chrome.line_h};
   }
 }