Rework the panel onto SI figures, labelled chips and one spacing scale

text.go

@@ -26,6 +26,30 @@ type textFace struct {
 	cellH  int
 	ascent int
 	cache  map[rune]*glyph
+
+	// Where a line of text starts and stops as far as the eye is concerned: the
+	// top of a digit or capital, down to the baseline. The cell is taller at
+	// both ends, reserving space above for accents nothing here uses and below
+	// for descenders, which hang past the line without being read as part of
+	// it. Laying out by the cell therefore puts visibly more air around text
+	// than around a bordered box the same distance away.
+	capTop int
+	lineH  int
+}
+
+// Digits carry no ascender or descender, so the first row one marks is the top
+// of the line and the baseline is the bottom.
+func (t *textFace) measureLine() error {
+	g := t.glyph('0')
+	for y := 0; y < g.h; y++ {
+		for x := 0; x < g.w; x++ {
+			if g.cov[y*g.w+x] != 0 {
+				t.capTop, t.lineH = y, t.ascent-y
+				return nil
+			}
+		}
+	}
+	return fmt.Errorf("font rasterised no ink for a digit")
 }
 
 func loadFace(bold bool, sizePx float64) (*textFace, error) {
@@ -54,13 +78,17 @@ func loadFace(bold bool, sizePx float64) (*textFace, error) {
 	if !ok {
 		return nil, fmt.Errorf("font has no digit glyphs")
 	}
-	return &textFace{
+	t := &textFace{
 		face:   face,
 		cellW:  adv.Ceil(),
 		cellH:  (m.Ascent + m.Descent).Ceil(),
 		ascent: m.Ascent.Ceil(),
 		cache:  make(map[rune]*glyph),
-	}, nil
+	}
+	if err := t.measureLine(); err != nil {
+		return nil, err
+	}
+	return t, nil
 }
 
 // Rasterises once per rune and keeps the coverage mask, since the same few