showcase: extend rank history to 32 weeks, trim leading empty columns

Previously the graph was fixed at 5 weekly data points.  Now:

- rankHistoryPoints raised from 5 → 32 so the store accumulates up to 32
  weekly snapshots and each project carries 32 history entries.
- GenerateRankHistorySVG trims all leading all-zero columns before layout,
  so the graph only spans from the oldest week that any project actually
  has data — no empty space on the left when history is short.
- displayPoints (trimmed width) drives all layout math: xPos(), xLabels,
  and the X-axis loop; numPoints is only used for the reversal placement.
- xLabelStep() thins X-axis text labels when many columns are visible
  (step 1 for ≤6, 2 for ≤12, 4 for ≤24, 8 for >24 columns) while still
  drawing a grid line at every column and always labelling 'now'.
- Test: give alpha a Spot at '4w' so trimming keeps that column and the
  time-axis label test still finds '4w ago' in the output.
- New TestXLabelStep_ReturnsSensibleSteps covers the new helper.

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

1 files changed, 27 insertions, 1 deletions

diff --git a/internal/showcase/rank_history_svg_test.go b/internal/showcase/rank_history_svg_test.go
index 06aab4c..d086a7d 100644
--- a/internal/showcase/rank_history_svg_test.go
+++ b/internal/showcase/rank_history_svg_test.go
@@ -16,7 +16,7 @@ func makeTestSummaries() []ProjectSummary {
 				{Spot: 2, Anchor: "1w", SnapshotDate: "2026-05-20"},
 				{Spot: 2, Anchor: "2w", SnapshotDate: "2026-05-13"},
 				{Spot: 3, Anchor: "3w", SnapshotDate: "2026-05-06"},
-				{Spot: 0, Anchor: "4w"}, // missing snapshot week
+				{Spot: 4, Anchor: "4w", SnapshotDate: "2026-04-29"}, // oldest data point
 			},
 		},
 		{
@@ -239,6 +239,32 @@ func TestGenerateRankHistorySVG_PartialRankHistory(t *testing.T) {
 	require(strings.Contains(svg, `"name":"single-entry"`), "SVG should include 'single-entry' project")
 }
 
+// TestXLabelStep_ReturnsSensibleSteps verifies the X-axis label density
+// function thins out labels as the number of display columns grows.
+func TestXLabelStep_ReturnsSensibleSteps(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		displayPoints int
+		wantMax       int // step must be ≤ wantMax
+	}{
+		{5, 1},
+		{10, 2},
+		{20, 4},
+		{32, 8},
+	}
+
+	for _, tc := range tests {
+		got := xLabelStep(tc.displayPoints)
+		if got > tc.wantMax {
+			t.Errorf("xLabelStep(%d) = %d, want ≤ %d", tc.displayPoints, got, tc.wantMax)
+		}
+		if got <= 0 {
+			t.Errorf("xLabelStep(%d) = %d, want > 0", tc.displayPoints, got)
+		}
+	}
+}
+
 // TestGridStep_ReturnsSensibleSteps ensures the Y-axis step function keeps
 // the graph legible across various project counts.
 func TestGridStep_ReturnsSensibleSteps(t *testing.T) {