ttools: report SliderSpecifier value without spurious precision

ttools/src/main/uk/ac/starlink/ttools/plot2/PlotUtil.java

@@ -658,6 +658,41 @@ public static String formatNumber( double value, String baseFmt,
         return fmt.format( value );
     }
 
+    /**
+     * Formats a number so that it presents a number of significant figures
+     * corresponding to a supplied small difference.
+     * The idea is that the output should be compact, but that applying
+     * this function to value and value+epsilon should give visibly
+     * different results.  The number of significant figures is determined
+     * by epsilon, not further rounded (trailing zeroes are not truncated).
+     *
+     * @param   value   value to format
+     * @param   epsilon   small value
+     * @return  formatted value
+     */
+    public static String formatNumber( double value, double epsilon ) {
+        epsilon = Math.abs( epsilon );
+
+        /* Work out the number of significant figures. */
+        double aval = Math.abs( value );
+        int nsf =
+            Math.max( 0, (int) Math.round( -Math.log10( epsilon / aval ) ) );
+
+        /* Return a formatted string on this basis. */
+        if ( aval >= 1e6 || aval <= 1e-4 ) {
+            return formatNumber( value, "0.#E0", nsf );
+        }
+        else if ( epsilon >= 0.9 ) {
+            return Long.toString( (long) Math.round( value ) );
+        }
+        else {
+            int ndp = (int) Math.round( Math.max( 0, -Math.log10( epsilon ) ) );
+            return ndp == 0
+                 ? Long.toString( (long) Math.round( value ) )
+                 : formatNumber( value, "0.0", ndp );
+        }
+    }
+
     /**
      * Returns the rectangle that results from removing the insets from
      * a given rectangle.

ttools/src/main/uk/ac/starlink/ttools/plot2/config/SliderSpecifier.java

@@ -16,6 +16,7 @@
 import javax.swing.JTextField;
 import javax.swing.event.ChangeListener;
 import uk.ac.starlink.ttools.gui.ResourceIcon;
+import uk.ac.starlink.ttools.plot2.PlotUtil;
 
 /**
  * Double value specifier that uses a slider to choose a value in the
@@ -197,7 +198,33 @@ public double getTextValue() {
      * @return  slider value
      */
     public double getSliderValue() {
-        return scale( slider_.getValue() );
+        int i0 = slider_.getValue();
+        double d0 = scale( i0 );
+
+        /* For cosmetic reasons, try to round the value to a round number
+         * corresponding to the nearest pixel so that reporting the
+         * value as text does not include spurious (and ugly) precision.
+         * We do it by formatting the value using a pixel-sized value delta,
+         * and turning that formatted value back into a number. */
+        int npix = slider_.getOrientation() == JSlider.HORIZONTAL
+                 ? slider_.getWidth()
+                 : slider_.getHeight();
+        if ( npix > 10 ) {
+            int iPixStep = ( slider_.getMaximum() - slider_.getMinimum() )
+                         / npix;
+            double dPixStep = Math.abs( scale( i0 + iPixStep ) - d0 );
+            if ( dPixStep > 0 ) {
+                String numstr = PlotUtil.formatNumber( d0, dPixStep );
+                try {
+                    return Double.parseDouble( numstr );
+                }
+                catch ( NumberFormatException e ) {
+                }
+            }
+        }
+
+        /* If something went wrong, it's OK to use the exact value. */
+        return d0;
     }
 
     /**

ttools/src/main/uk/ac/starlink/ttools/plot2/geom/PlaneSurface.java

@@ -500,31 +500,7 @@ public static String formatPosition( Axis axis, double dpos ) {
 
         /* Work out pixel size in data coordinates by looking at the
          * data position of a point two pixels away. */
-        double dp2 =
-            axis.graphicsToData( (int) axis.dataToGraphics( dpos ) + 2 );
-        double prec = Math.abs( dp2 - dpos ) / 2.;
-
-        /* Work out the number of significant figures. */
-        double aval = Math.abs( dpos );
-        int nsf =
-            Math.max( 0, (int) Math.round( -Math.log10( prec / aval ) ) );
-
-        /* Return a formatted string on this basis. */
-        if ( aval >= 1e6 || aval <= 1e-4 ) {
-            return PlotUtil.formatNumber( dpos, "0.#E0", nsf );
-        }
-        else if ( prec >= 0.9 ) {
-            return Long.toString( (long) Math.round( dpos ) );
-        }
-        else {
-            int ndp =
-                (int) Math.round( Math.max( 0, -Math.log10( prec ) ) );
-            if ( ndp == 0 ) {
-                return Long.toString( (long) Math.round( dpos ) );
-            }
-            else {
-                return PlotUtil.formatNumber( dpos, "0.0", ndp );
-            }
-        }
+        double dp2 = axis.graphicsToData( axis.dataToGraphics( dpos ) + 2 );
+        return PlotUtil.formatNumber( dpos, Math.abs( dp2 - dpos ) / 2. );
     }
 }