figure: size and reset a Live without drawing a frame

live.go

@@ -244,27 +244,47 @@ func (l *Live) TrackRows(on bool) *Live {
 //
 // Resizing to the size it already has is not an error and draws nothing.
 func (l *Live) Resize(w, h int) error {
+	if changed, err := l.setSize(w, h); err != nil || !changed {
+		return err
+	}
+	return l.Draw()
+}
+
+// SetSize is [Live.Resize] without the frame: the chart takes the new size
+// and draws it on the next [Live.Draw].
+//
+// It is for a caller that coalesces its frames — a widget that is laid out
+// and then redrawn for another reason in the same turn, or one that is not on
+// screen at all, would otherwise paint the whole chart once for the size and
+// once more for the reason.
+func (l *Live) SetSize(w, h int) error {
+	_, err := l.setSize(w, h)
+	return err
+}
+
+// setSize is SetSize, reporting whether the size changed.
+func (l *Live) setSize(w, h int) (bool, error) {
 	if l.b == nil {
-		return errors.New("figure: Resize on a closed Live")
+		return false, errors.New("figure: Resize on a closed Live")
 	}
 	if w <= 0 || h <= 0 {
-		return fmt.Errorf("figure: chart size %dx%d is not positive", w, h)
+		return false, fmt.Errorf("figure: chart size %dx%d is not positive", w, h)
 	}
 	if w == l.width && h == l.height {
-		return nil
+		return false, nil
 	}
 	l.width, l.height = w, h
 	l.chart.Width, l.chart.Height = w, h
 	l.chart.Theme = l.p.themeFor(w, h)
 	if r, ok := l.b.(ir.Resizer); ok {
 		if err := r.Resize(ir.Surface{WidthPx: w, HeightPx: h, DPR: l.dpr}); err != nil {
-			return err
+			return false, err
 		}
 	}
 	// A frame of a different size is not comparable with the last one: every
 	// coordinate in it moved, so there is no damage to compute.
 	l.drawn = false
-	return l.Draw()
+	return true, nil
 }
 
 // Size reports the surface's current size in device-independent pixels.
@@ -585,6 +605,17 @@ func (l *Live) PanBy(dx, dy float64) error {
 // Autoscale releases every zoom and pan, so the axes come from the data again,
 // and redraws. It is the "reset view" every interactive chart needs.
 func (l *Live) Autoscale() error {
+	l.ResetView()
+	return l.Draw()
+}
+
+// ResetView is [Live.Autoscale] without the frame: every zoom and pan is
+// released, and the next [Live.Draw] shows the data's own extent.
+//
+// A caller that releases the view only to set another one straight after —
+// linked charts fitted to a common range — would otherwise draw the released
+// view first, a frame nobody sees.
+func (l *Live) ResetView() {
 	for _, p := range l.idx.Panels() {
 		// A fixed coord's domains were never moved, and releasing them would
 		// retrain them to the data and drop the grid's pinned ticks.
@@ -596,7 +627,6 @@ func (l *Live) Autoscale() error {
 		autoscale(p.Y2)
 		autoscale(p.X2)
 	}
-	return l.Draw()
 }
 
 // Select reports the rows under a device-space rectangle, firing one [Select]

live_test.go

@@ -214,3 +214,60 @@ type opaqueLayer struct{}
 func (opaqueLayer) Train(geom.Training) error                  { return nil }
 func (opaqueLayer) Build(ir.Backend, geom.Frame) error         { return nil }
 func (opaqueLayer) Legend(geom.Frame) (geom.LegendEntry, bool) { return geom.LegendEntry{}, false }
+
+// ResetView releases the zoom as Autoscale does, but leaves the frame to the
+// next Draw.
+func TestResetViewDrawsNothingUntilDraw(t *testing.T) {
+	_, live, rec := livePlot(t)
+	panel := live.Index().Panels()[0]
+	lo0, hi0 := panel.X.Domain()
+
+	sel := ir.Rect{
+		Min: ir.Point{X: panel.X.Map(1), Y: panel.Y.Map(6)},
+		Max: ir.Point{X: panel.X.Map(3), Y: panel.Y.Map(2)},
+	}
+	if err := live.ZoomTo(sel); err != nil {
+		t.Fatal(err)
+	}
+	calls := len(rec.Calls)
+
+	live.ResetView()
+	if len(rec.Calls) != calls {
+		t.Errorf("ResetView drew %d calls, want none", len(rec.Calls)-calls)
+	}
+	if err := live.Draw(); err != nil {
+		t.Fatal(err)
+	}
+	if len(rec.Calls) == calls {
+		t.Error("the Draw after ResetView painted nothing")
+	}
+	if lo, hi := live.Index().Panels()[0].X.Domain(); lo != lo0 || hi != hi0 {
+		t.Errorf("the domain is %v..%v after ResetView, want the data's %v..%v", lo, hi, lo0, hi0)
+	}
+}
+
+// SetSize takes the size as Resize does, but leaves the frame to the next
+// Draw.
+func TestSetSizeDrawsNothingUntilDraw(t *testing.T) {
+	_, live, rec := livePlot(t)
+	calls := len(rec.Calls)
+
+	if err := live.SetSize(500, 320); err != nil {
+		t.Fatal(err)
+	}
+	if len(rec.Calls) != calls {
+		t.Errorf("SetSize drew %d calls, want none", len(rec.Calls)-calls)
+	}
+	if w, h := live.Size(); w != 500 || h != 320 {
+		t.Errorf("the size is %dx%d, want 500x320", w, h)
+	}
+	if err := live.Draw(); err != nil {
+		t.Fatal(err)
+	}
+	if len(rec.Calls) == calls {
+		t.Error("the Draw after SetSize painted nothing")
+	}
+	if err := live.SetSize(0, 320); err == nil {
+		t.Error("SetSize took a width of zero")
+	}
+}