demo: Strings on the Y-axis via custom tick formatter (issue #884)

xchart-demo/src/main/java/org/knowm/xchart/standalone/issues/TestForIssue884.java

@@ -0,0 +1,66 @@
+package org.knowm.xchart.standalone.issues;
+
+import java.util.Arrays;
+import java.util.List;
+import org.knowm.xchart.SwingWrapper;
+import org.knowm.xchart.XYChart;
+import org.knowm.xchart.XYChartBuilder;
+import org.knowm.xchart.XYSeries.XYSeriesRenderStyle;
+
+/**
+ * Reproducer / how-to for https://github.com/knowm/XChart/issues/884
+ *
+ * <p>"For a chart I need on the Y-Axis Strings instead of Numbers."
+ *
+ * <p>XChart does not transpose arbitrary charts, but any axes-based chart can show Strings on the
+ * Y-axis by plotting numeric category indices (0, 1, 2, ...) and supplying a custom Y-axis tick
+ * label formatter that maps each numeric tick position back to a String. This works for XYChart,
+ * line/scatter, etc. — not just bar charts.
+ */
+public class TestForIssue884 {
+
+  /** The String labels we want to see on the Y-axis instead of numbers. */
+  private static final List<String> CATEGORIES =
+      Arrays.asList("Apples", "Bananas", "Cherries", "Dates", "Elderberries");
+
+  public static void main(String[] args) {
+
+    new SwingWrapper<>(getChart()).displayChart();
+  }
+
+  /** Constructs and returns the chart without launching a window (headless-safe). */
+  public static XYChart getChart() {
+
+    XYChart chart =
+        new XYChartBuilder()
+            .width(800)
+            .height(600)
+            .title("Strings on the Y-Axis (issue 884)")
+            .xAxisTitle("Value")
+            .yAxisTitle("Fruit")
+            .build();
+
+    chart.getStyler().setDefaultSeriesRenderStyle(XYSeriesRenderStyle.Scatter);
+
+    // Y values are simply the category indices: 0..CATEGORIES.size()-1
+    double[] xData = {12, 7, 19, 4, 15};
+    double[] yData = {0, 1, 2, 3, 4};
+    chart.addSeries("counts", xData, yData);
+
+    // The key: map each numeric Y tick position to its String label. Non-integer tick positions
+    // (the formatter may be asked about fractional grid lines) are blanked out so only the
+    // category rows are labelled.
+    chart
+        .getStyler()
+        .setYAxisTickLabelsFormattingFunction(
+            value -> {
+              int index = (int) Math.round(value);
+              if (Math.abs(value - index) < 1e-9 && index >= 0 && index < CATEGORIES.size()) {
+                return CATEGORIES.get(index);
+              }
+              return "";
+            });
+
+    return chart;
+  }
+}