make_hero.py: compute widest band per-quarter and describe lower edge from data

assets/make_hero.py

@@ -77,14 +77,24 @@ def band(lo, hi):
         )
         return f"M{up} L{dn} Z"
 
+    widths = [hi - lo for lo, hi in zip(lo90, hi90)]
+    w = widths.index(max(widths))
+    neg = [i for i, lo in enumerate(lo90) if lo < 0]
+    if not neg:
+        edge = "its lower edge stays positive throughout the forecast"
+    elif neg[0] == 0:
+        edge = "its lower edge is below zero throughout the forecast"
+    else:
+        edge = (
+            f"its lower edge is positive for the first "
+            f"{'quarter' if neg[0] == 1 else str(neg[0]) + ' quarters'} and dips "
+            f"below zero from {quarters[neg[0]]} onward"
+        )
     desc = (
         f"Fan chart. Posterior forecast of UK year-on-year real GDP growth from the "
         f"boe-svar model, {quarters[0]} to {quarters[-1]}, conditioned through 2026Q1. The "
         f"median path runs from {med[0]:+.2f}% to {med[-1]:+.2f}%. The 90% credible band "
-        f"is widest at {quarters[lo90.index(min(lo90))]}, spanning {min(lo90):+.2f}% to "
-        f"{max(hi90):+.2f}%, and the lower edge stays below zero throughout the forecast: "
-        f"the model does not rule out a contraction. This figure is the site's hero motif "
-        f"and uses the site's current-forecast artifact rather than the frozen validation run."
+        f"is widest at {quarters[w]}, spanning {lo90[w]:+.2f}% to {hi90[w]:+.2f}%; {edge}."
     )
 
     out = [

index.html

@@ -61,7 +61,7 @@ <h1 class="reveal" style="--d:1">
 <!-- hero-fan:begin -->
 <svg class="hero-fan" viewBox="0 0 640 400" preserveAspectRatio="xMidYMid meet" role="img" aria-labelledby="hero-fan-t hero-fan-d">
 <title id="hero-fan-t">boe-svar: UK real GDP growth forecast, median with 68% and 90% credible bands</title>
-<desc id="hero-fan-d">Fan chart. Posterior forecast of UK year-on-year real GDP growth from the boe-svar model, 2026Q2 to 2029Q2, conditioned through 2026Q1. The median path runs from +1.14% to +1.41%. The 90% credible band is widest at 2029Q2, spanning -0.67% to +3.77%; its lower edge is positive for the first three quarters and dips below zero from 2027Q1 onward: the model does not rule out a contraction.</desc>
+<desc id="hero-fan-d">Fan chart. Posterior forecast of UK year-on-year real GDP growth from the boe-svar model, 2026Q2 to 2029Q2, conditioned through 2026Q1. The median path runs from +1.14% to +1.41%. The 90% credible band is widest at 2029Q2, spanning -0.67% to +3.77%; its lower edge is positive for the first 3 quarters and dips below zero from 2027Q1 onward.</desc>
 <line class="hf-grid" x1="30.0" y1="302.3" x2="604.0" y2="302.3"/>
 <text class="hf-tick" x="30.0" y="295.3">-1%</text>
 <line class="hf-zero" x1="30.0" y1="242.7" x2="604.0" y2="242.7"/>