Add price-range zoom controls to the P/L chart

−/+ buttons (with a ±% readout) and a Fit button in the chart header widen
or narrow the underlying-price window the curve is sampled over, so you can
inspect a tight range around spot or zoom out to see the full payoff shape.
Bumped sampling density to 221 points.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/strategy.html

@@ -96,15 +96,25 @@
 
           <!-- P/L chart -->
           <div class="chart-card mb-3" x-show="legs.length > 0" x-cloak>
-            <div class="card-header d-flex align-items-center justify-content-between flex-wrap gap-2">
+            <div class="card-header d-flex align-items-center justify-content-between flex-wrap gap-3">
               <h3 class="card-title text-white mb-0">Profit / Loss vs. Underlying Price</h3>
-              <div class="d-flex align-items-center gap-2" style="min-width:320px;">
+              <div class="d-flex align-items-center flex-wrap gap-3">
+                <!-- price-range zoom -->
+                <div class="btn-group btn-group-sm" role="group" aria-label="Zoom price range">
+                  <button class="btn btn-outline-secondary" @click="zoomOut()" title="Zoom out — wider price range">−</button>
+                  <button class="btn btn-outline-secondary" disabled style="min-width:5rem" x-text="'±' + lastHalfPct + '%'"></button>
+                  <button class="btn btn-outline-secondary" @click="zoomIn()" title="Zoom in — narrower price range">+</button>
+                  <button class="btn btn-outline-secondary" @click="zoomFit()" title="Reset zoom">Fit</button>
+                </div>
+                <!-- time slider -->
+                <div class="d-flex align-items-center gap-2" style="min-width:300px;">
                   <span class="text-secondary small text-nowrap">Now</span>
-                <input type="range" class="form-range" min="0" :max="maxDTE" step="1" x-model.number="dteOffset" @input="scheduleRender()" style="min-width:160px;">
+                  <input type="range" class="form-range" min="0" :max="maxDTE" step="1" x-model.number="dteOffset" @input="scheduleRender()" style="min-width:150px;">
                   <span class="text-secondary small text-nowrap">Exp</span>
                   <span class="badge bg-blue-lt text-nowrap" x-text="dteLabel"></span>
                 </div>
               </div>
+            </div>
             <div class="card-body p-0">
               <div id="plChart" style="min-height:380px;" role="img" aria-label="Profit and loss diagram"></div>
             </div>
@@ -300,6 +310,7 @@
         // state
         symbol: '', symbols: [], spot: 0, legs: [],
         dteOffset: 0,
+        xZoom: 1, lastHalfPct: 0,
         refreshing: false, showManual: false, toast: '',
         manual: { side:'long', type:'call', qty:1, strike:null, expiry:'', entryPrice:null, ivPct:null },
         chart: null, _renderTimer: null,
@@ -517,6 +528,9 @@
           clearTimeout(this._renderTimer);
           this._renderTimer = setTimeout(() => this.renderChart(), 40);
         },
+        zoomIn()  { this.xZoom = Math.max(0.2, +(this.xZoom / 1.4).toFixed(3)); this.renderChart(); },
+        zoomOut() { this.xZoom = Math.min(8,   +(this.xZoom * 1.4).toFixed(3)); this.renderChart(); },
+        zoomFit() { this.xZoom = 1; this.renderChart(); },
 
         renderChart() {
           const legs = this.activeLegs, net = this.netCost;
@@ -528,10 +542,12 @@
           const strikes = legs.map(l=>l.strike);
           const minK = Math.min(...strikes), maxK = Math.max(...strikes);
           const span = maxK - minK;
-          const half = Math.max(spot*0.22, span*1.5, (maxK-spot)*1.4, (spot-minK)*1.4, 5);
+          const baseHalf = Math.max(spot*0.22, span*1.5, (maxK-spot)*1.4, (spot-minK)*1.4, 5);
+          const half = baseHalf * (this.xZoom || 1);
           const lo = Math.max(0.01, spot - half), hi = spot + half;
+          this.lastHalfPct = spot > 0 ? Math.round(half / spot * 100) : 0;
           const expAt = this.minDTE;
-          const N = 161;
+          const N = 221;
           // x-grid = evenly spaced points + the exact strike kinks (so payoff
           // vertices — butterfly peak, condor body, etc. — render sharply)
           const xGrid = [];