jfjoch_viewer: FFT frequency axis 1000x too low

DiffractionExperiment::GetImageTime() returns std::chrono::nanoseconds, but
the chart stored its count() as microseconds, so the sampling frequency came
out 1000x too small (a 6 ms image time gave a Nyquist of ~0.08 Hz instead of
~83 Hz). Keep the image time in seconds via duration<double>.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

viewer/charts/JFJochDatasetInfoChartView.cpp

@@ -184,10 +184,10 @@ void JFJochDatasetInfoChartView::loadValues(const std::vector<float> &input, int
 
     if (dataset != nullptr) {
         goniometer_axis = dataset->experiment.GetGoniometer();
-        image_time_us = dataset->experiment.GetImageTime().count();
+        image_time_s = std::chrono::duration<double>(dataset->experiment.GetImageTime()).count();
     } else {
         goniometer_axis = {};
-        image_time_us = {};
+        image_time_s = {};
     }
 
     curr_image = image;
@@ -711,7 +711,7 @@ void JFJochDatasetInfoChartView::contextMenuEvent(QContextMenuEvent *event) {
     actShowFFT->setCheckable(true);
     actShowFFT->setChecked(m_showFFT);
     // Require valid sampling interval
-    actShowFFT->setEnabled(!values.empty() && image_time_us > 0.0);
+    actShowFFT->setEnabled(!values.empty() && image_time_s > 0.0);
 #endif
 
     QAction *chosen = menu.exec(event->globalPos());
@@ -961,7 +961,7 @@ void JFJochDatasetInfoChartView::onHoverLoadTimeout() {
 void JFJochDatasetInfoChartView::buildFFTChart() {
     const size_t N = values.size();
 
-    if (N == 0 || !image_time_us.has_value() || image_time_us <= 0.0) {
+    if (N == 0 || !image_time_s.has_value() || image_time_s <= 0.0) {
         return;
     }
 
@@ -1001,7 +1001,7 @@ void JFJochDatasetInfoChartView::buildFFTChart() {
     m_fftMagnitudes.resize(static_cast<size_t>(nComplex));
     m_fftFrequenciesHz.resize(static_cast<size_t>(nComplex));
 
-    const double dt = image_time_us.value() * 1e-6;             // seconds per sample
+    const double dt = image_time_s.value();             // seconds per sample
     const double fs = 1.0 / dt;                         // sampling frequency
     const double df = fs / static_cast<double>(n);      // frequency resolution
 

viewer/charts/JFJochDatasetInfoChartView.h

@@ -64,7 +64,7 @@ class JFJochDatasetInfoChartView : public QChartView {
     void setHoverLineY(double y, double snap_to_y);
 
     std::optional<GoniometerAxis> goniometer_axis;
-    std::optional<float> image_time_us;
+    std::optional<double> image_time_s;
 
     void updateChart();