web/sparkline: guard days[i] against shorter days slice in RenderChart

internal/web/sparkline/sparkline.go

@@ -141,7 +141,8 @@ type ChartOpts struct {
 // RenderChart returns an inline SVG <svg> with up to two y-axes,
 // one polyline per series, hover-dot per data point, and X-axis
 // labels at start / midpoint / end. With no series or empty
-// series, renders a faint baseline + EmptyLabel centred.
+// series, renders a faint baseline + EmptyLabel centred. Points
+// beyond len(days) are ignored.
 func RenderChart(series []Series, days []time.Time, opts ChartOpts) template.HTML {
 	if opts.Width <= 0 {
 		opts.Width = 600
@@ -246,6 +247,9 @@ func RenderChart(series []Series, days []time.Time, opts ChartOpts) template.HTM
 			seg.Reset()
 		}
 		for i, v := range s.Points {
+			if i >= len(days) {
+				break
+			}
 			if math.IsNaN(v) {
 				flush()
 				continue