chart: label a narrow axis in decimals

A span of 1.2 degrees rounded to whole degrees printed "15" against both the
middle and bottom rows, so the axis asserted that two rows drawn at different
heights were the same temperature. It reads as a rendering fault, and the scale
becomes useless exactly when the chart is at its most magnified.

Below a two degree span the labels carry one decimal. Two degrees is where
five rows put adjacent labels half a degree apart, which still rounds apart;
below that they collide. A decimal label that would not fit the gutter -- well
below freezing, where the minus sign costs a column -- falls back to whole
degrees, which is wrong but legible and beats shearing the column.

The gutter widens by one to hold "16.0°".

1 files changed, 40 insertions, 7 deletions

diff --git a/internal/render/chart.go b/internal/render/chart.go
index cc20ca9..ce41364 100644
--- a/internal/render/chart.go
+++ b/internal/render/chart.go
@@ -28,7 +28,9 @@ type column struct {
 // hourly points and would otherwise wrap into mush.
 func (x ctx) chart(v View) []string {
 	height := x.cfg.GraphHeight
-	const gutter = 5 // "NN°" label plus the axis rule
+	// Wide enough for the widest label the axis can produce -- "16.0°" when the
+	// range is too narrow for whole degrees -- plus the axis rule.
+	const gutter = 6
 	cols := x.width - gutter - 1
 	if cols < 8 {
 		cols = 8
@@ -82,18 +84,25 @@ func (x ctx) chart(v View) []string {
 		}
 	}
 
+	// Whole degrees cannot label a narrow range: a span of 1.2 degrees makes the
+	// middle and bottom rows both read "15", which says the two rows are the
+	// same temperature when they are not. Below the threshold, use a decimal.
+	// Falls back to whole degrees if a decimal label would not fit the gutter,
+	// which only happens well below freezing.
+	axisLabel := tempLabeller(lo, hi, gutter-1)
+
 	out := []string{""}
 	for r := 0; r < height; r++ {
 		full := (height - r) * 2
 		value := lo + (hi-lo)*float64(height-1-r)/float64(height-1)
-		label := "    "
+		label := strings.Repeat(" ", gutter-1)
 		switch {
 		case r == 0:
-			label = x.c(TempStyle(x.celsius(hi)), PadLeft(fmt.Sprintf("%d°", Deg(hi)), 4))
+			label = x.c(TempStyle(x.celsius(hi)), PadLeft(axisLabel(hi), gutter-1))
 		case r == height-1:
-			label = x.c(TempStyle(x.celsius(lo)), PadLeft(fmt.Sprintf("%d°", Deg(lo)), 4))
+			label = x.c(TempStyle(x.celsius(lo)), PadLeft(axisLabel(lo), gutter-1))
 		case height >= 5 && r == height/2:
-			label = x.c(TempStyle(x.celsius(value)), PadLeft(fmt.Sprintf("%d°", Deg(value)), 4))
+			label = x.c(TempStyle(x.celsius(value)), PadLeft(axisLabel(value), gutter-1))
 		}
 		cells := make([]Cell, 0, len(drawn))
 		for _, d := range drawn {
@@ -109,7 +118,7 @@ func (x ctx) chart(v View) []string {
 		out = append(out, label+x.c(Dim, "│")+Paint(cells, x.c))
 	}
 
-	note := fmt.Sprintf("%d-%d°", Deg(lo), Deg(hi))
+	note := axisLabel(lo) + "-" + axisLabel(hi)
 	if len(groups) < len(v.Rows) {
 		note += fmt.Sprintf("  %.0f%s", float64(len(v.Rows))/float64(len(groups)), x.cat.Word("h_per_col"))
 	}
@@ -124,7 +133,7 @@ func (x ctx) chart(v View) []string {
 		for i, d := range drawn {
 			series[i] = d.rain
 		}
-		out = append(out, x.c(Dim, PadLeft(x.cat.Word("rain_row"), 4)+"│")+
+		out = append(out, x.c(Dim, PadLeft(x.cat.Word("rain_row"), gutter-1)+"│")+
 			x.c(Cyan, spark(series))+
 			x.c(Dim, fmt.Sprintf("  %s %.1fmm", x.cat.Word("max"), maxRain)))
 	}
@@ -203,3 +212,27 @@ func maxInt(a, b int) int {
 func ceilDiv(a, b int) int { return (a + b - 1) / b }
 
 var _ = openmeteo.Row{}
+
+// minSpanForWholeDegrees is the range below which whole-degree axis labels stop
+// distinguishing rows. With five rows a 2-degree span puts adjacent labels half
+// a degree apart, which still rounds to distinct values; below that they
+// collide.
+const minSpanForWholeDegrees = 2.0
+
+// tempLabeller returns a formatter for the y-axis, choosing precision from the
+// range so that labels stay distinct, and never exceeding maxWidth cells.
+func tempLabeller(lo, hi float64, maxWidth int) func(float64) string {
+	whole := func(v float64) string { return fmt.Sprintf("%d°", Deg(v)) }
+	if hi-lo >= minSpanForWholeDegrees {
+		return whole
+	}
+	decimal := func(v float64) string { return fmt.Sprintf("%.1f°", v) }
+	// A decimal label is two characters longer; if that will not fit, whole
+	// degrees are wrong but legible, which beats a sheared column.
+	for _, v := range []float64{lo, hi} {
+		if DisplayWidth(decimal(v)) > maxWidth {
+			return whole
+		}
+	}
+	return decimal
+}