test: regression test for issue #707 (last bar label overwritten in CategoryChart)

xchart/src/test/java/org/knowm/xchart/CategoryChartTest.java

@@ -6,13 +6,16 @@
 import static org.knowm.xchart.style.Styler.ChartTheme.GGPlot2;
 import static org.knowm.xchart.style.Styler.ChartTheme.XChart;
 
+import java.awt.Color;
+import java.awt.image.BufferedImage;
 import java.util.*;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 import org.knowm.xchart.custom.CustomGraphic;
 import org.knowm.xchart.custom.CustomTheme;
 import org.knowm.xchart.internal.series.Series;
 import org.knowm.xchart.style.Styler;
+import org.knowm.xchart.style.Styler.ChartTheme;
 
 public class CategoryChartTest {
 
@@ -245,4 +248,75 @@ void paint() {
           .contains(chart.getStyler().getDefaultSeriesRenderStyle());
     }
   }
+
+  /**
+   * Regression test for <a href="https://github.com/knowm/XChart/issues/707">issue 707</a>.
+   *
+   * <p>The last bar in a Bar-style CategoryChart was drawn twice (overwriting its label) because
+   * the bar loop reused the shared {@code path} variable that {@code closePath()} consumes after
+   * the loop. The fix uses a local {@code barPath} so {@code path} stays {@code null} for bar
+   * series and {@code closePath()} becomes a no-op.
+   *
+   * <p>To catch a visual regression we render to a {@link BufferedImage} and compare
+   * pixel-diff counts (labels-on minus labels-off) between:
+   *
+   * <ol>
+   *   <li>A 5-bar chart with all values present (all 5 labels rendered).
+   *   <li>The same 5-bar chart where the last value is {@link Double#NaN} (4 labels rendered,
+   *       same x/y axis extents and bar positions).
+   * </ol>
+   *
+   * <p>When the bug is reintroduced the 5th label is overwritten by the bar fill, making its
+   * diff indistinguishable from the NaN chart's diff. The assertion {@code diff5 > diffNaN}
+   * therefore fails precisely when and only when the 5th label is missing.
+   */
+  @Test
+  void issue707LastBarLabelIsVisibleAndNotOverwritten() throws Exception {
+    List<Double> xData = Arrays.asList(0.0, 1.0, 2.0, 3.0, 4.0);
+
+    // Chart A — all 5 values present; should render 5 labels.
+    CategoryChart chart5On = buildIssue707Chart(xData, Arrays.asList(2.0, 1.5, 4.0, 3.77, 2.5), true);
+    CategoryChart chart5Off = buildIssue707Chart(xData, Arrays.asList(2.0, 1.5, 4.0, 3.77, 2.5), false);
+
+    // Chart B — last value is NaN so bar 5 is skipped; only 4 labels are rendered.
+    CategoryChart chartNaNOn =
+        buildIssue707Chart(
+            xData, Arrays.asList(2.0, 1.5, 4.0, 3.77, Double.NaN), true);
+    CategoryChart chartNaNOff =
+        buildIssue707Chart(
+            xData, Arrays.asList(2.0, 1.5, 4.0, 3.77, Double.NaN), false);
+
+    int diff5 = countDiffPixels(BitmapEncoder.getBufferedImage(chart5On), BitmapEncoder.getBufferedImage(chart5Off));
+    int diffNaN = countDiffPixels(BitmapEncoder.getBufferedImage(chartNaNOn), BitmapEncoder.getBufferedImage(chartNaNOff));
+
+    // When the fix is in place, diff5 > diffNaN because the 5th label contributes pixels.
+    // When the bug is reintroduced, the 5th label is overwritten and diff5 == diffNaN.
+    assertThat(diff5)
+        .as(
+            "labels-on/off pixel diff for 5-bar chart (%d) must exceed that of the same chart "
+                + "with NaN last bar (%d); equality means the 5th label was overwritten",
+            diff5,
+            diffNaN)
+        .isGreaterThan(diffNaN);
+  }
+
+  private CategoryChart buildIssue707Chart(
+      List<Double> xData, List<Double> yData, boolean labelsVisible) {
+    CategoryChart chart = new CategoryChart(800, 600, ChartTheme.Matlab);
+    chart.getStyler().setLabelsVisible(labelsVisible);
+    chart.getStyler().setLabelsFontColorAutomaticEnabled(false);
+    chart.getStyler().setLabelsFontColor(new Color(255, 0, 255));
+    chart.addSeries("y(x)", xData, yData);
+    return chart;
+  }
+
+  private int countDiffPixels(BufferedImage a, BufferedImage b) {
+    int count = 0;
+    for (int x = 0; x < a.getWidth(); x++) {
+      for (int y = 0; y < a.getHeight(); y++) {
+        if (a.getRGB(x, y) != b.getRGB(x, y)) count++;
+      }
+    }
+    return count;
+  }
 }