diff --git a/sfm/report_pdf.py b/sfm/report_pdf.py
index 2f57f63..2a30939 100644
--- a/sfm/report_pdf.py
+++ b/sfm/report_pdf.py
@@ -129,6 +129,7 @@ class ReportData:
     histogram_stop_str:     Optional[str] = None
     histogram_n_intervals:  Optional[float] = None     # 4.00
     histogram_interval_size: Optional[str] = None      # "1 minute"
+    histogram_interval_size_s: Optional[float] = None  # 60.0 — numeric seconds, used to derive interval_times
     histogram_interval_times: list[str] = field(default_factory=list)  # per-interval timestamps for x-axis
 
     # Peak Vector Sum metadata (histograms show absolute date+time)
@@ -265,6 +266,7 @@ def gather_report_data(
             rd.histogram_stop_str    = hist_block.get("stop")
             rd.histogram_n_intervals = hist_block.get("n_intervals")
             rd.histogram_interval_size = hist_block.get("interval_size")
+            rd.histogram_interval_size_s = hist_block.get("interval_size_s")
             rd.histogram_interval_times = hist_block.get("interval_times") or []
 
     # ── Waveform samples — from the .h5 via the existing helper ──