Zoomer: base zoom calculations on pixels

Previously the zoomer would perform the scroll-zoom and panning
calculations in data coordinates. This is identical to pixel coordinates
for linear axes, but for e.g. log axes this leads to strange behaviour
since it can result in negative data values.

This changes the Pan and pan and scroll behaviour, the rectangle zoom
was naturally always using screen coordinates anyway, so this should
also be more consistent.

chartfx-chart/src/main/java/io/fair_acc/chartfx/plugins/Zoomer.java

@@ -859,15 +859,19 @@ private void panChart(final Chart chart, final Point2D mouseLocation) {
 
             final Side side = axis.getSide();
 
-            final double prevData = axis.getValueForDisplay(side.isHorizontal() ? oldMouseX : oldMouseY);
-            final double newData = axis.getValueForDisplay(side.isHorizontal() ? newMouseX : newMouseY);
-            final double offset = prevData - newData;
-
             final boolean allowsShift = side.isHorizontal() ? getAxisMode().allowsX() : getAxisMode().allowsY();
             if (!hasBoundedRange(axis) && allowsShift) {
+                final double offset = (side.isHorizontal() ? oldMouseX : oldMouseY) - (side.isHorizontal() ? newMouseX : newMouseY);
                 axis.setAutoRanging(false);
                 // shift bounds
-                axis.set(axis.getMin() + offset, axis.getMax() + offset);
+                final double newMin = axis.getValueForDisplay(0 + offset);
+                final double newMax = axis.getValueForDisplay(axis.getLength() + offset);
+                if (side.isHorizontal()) {
+                    axis.set(newMin, newMax);
+                } else {
+                    axis.set(newMax, newMin);
+                }
+                axis.updateCachedTransforms();
             }
         }
         previousMouseLocation = mouseLocation;
@@ -948,6 +952,7 @@ private void performZoom(Entry<Axis, ZoomState> zoomStateEntry, final boolean is
             if (!hasBoundedRange(axis)) {
                 // only update if this axis is not bound to another (e.g. auto-range) managed axis)
                 axis.set(zoomState.zoomRangeMin, zoomState.zoomRangeMax);
+                axis.updateCachedTransforms();
             }
         }
 
@@ -1155,14 +1160,22 @@ private static void zoomOnAxis(final Axis axis, final ScrollEvent event) {
         final boolean isHorizontal = axis.getSide().isHorizontal();
 
         final double mousePos = isHorizontal ? event.getX() : event.getY();
-        final double posOnAxis = axis.getValueForDisplay(mousePos);
-        final double max = axis.getMax();
-        final double min = axis.getMin();
+        final double max = axis.getLength();
         final double scaling = isZoomIn ? 0.9 : 1 / 0.9;
-        final double diffHalf1 = scaling * Math.abs(posOnAxis - min);
-        final double diffHalf2 = scaling * Math.abs(max - posOnAxis);
+        final double minDisplay;
+        final double maxDisplay;
+        if (isHorizontal) {
+            minDisplay = (1 - scaling) * mousePos;
+            maxDisplay = mousePos + scaling * (max - mousePos);
+        } else {
+            maxDisplay = (1 - scaling) * mousePos;
+            minDisplay = mousePos + scaling * (max - mousePos);
+        }
+        final double newMin = axis.getValueForDisplay(minDisplay);
+        final double newMax = axis.getValueForDisplay(maxDisplay);
 
-        axis.set(posOnAxis - diffHalf1, posOnAxis + diffHalf2);
+        axis.set(newMin, newMax);
+        axis.updateCachedTransforms();
 
         axis.forceRedraw();
     }

chartfx-samples/src/main/java/io/fair_acc/sample/chart/LogAxisSample.java

@@ -30,9 +30,7 @@ public Node getChartPanel(final Stage primaryStage) {
         // yAxis.setLogarithmBase(2);
 
         final XYChart chart = new XYChart(xAxis, yAxis);
-        final Zoomer zoomer = new Zoomer();
-        zoomer.setPannerEnabled(false); // not recommended (ie. Axes do not support complex numbers, e.g. 'log(-1)')
-        chart.getPlugins().add(zoomer); // zoom around
+        chart.getPlugins().add(new Zoomer()); // zoom around
         chart.getPlugins().add(new EditAxis()); // manually modify axis
 
         root.getChildren().add(chart);