diff --git a/src/corchestrate/chart.py b/src/corchestrate/chart.py
index c94cf15..b0fd356 100644
--- a/src/corchestrate/chart.py
+++ b/src/corchestrate/chart.py
@@ -43,6 +43,9 @@ def plot_over_time(input_data: list[ChartData]):
             if current_measurement is None:
                 continue
 
+            # Ensure min_datetime is minimum.
+            assert current_measurement.measured_at >= min_datetime
+
             if current_measurement.measured_at == min_datetime:
                 indexed_input.extracted_heart_rate.append(
                     current_measurement.heart_rate)