Fix #712: guard against zero/negative plot area in Plot_AxesChart

[timmolter]

Problem

When a XChartPanel is resized to be very narrow — especially with multiple Y-axis groups — each axis column consumes horizontal space. Once the total axis width exceeds the chart width, the remaining plot area becomes zero or negative. Painting with that degenerate geometry caused a repeating NullPointerException deep inside PlotContent_XY.doPaint().

Fixes #712.

Root Cause

Plot_AxesChart.paint() computed width = xAxisBounds.getWidth() and height = yAxisBounds.getHeight() and passed them straight to new Rectangle2D.Double(...). When width <= 0 or height <= 0, every downstream renderer (PlotContent_XY, Legend_, etc.) received a rectangle with invalid dimensions, causing NPEs in AWT path operations.

The existing guard in PlotContent_.paint() (bounds.getWidth() < 30) handles near-zero cases but is insufficient when the bounds goes fully non-positive at the Plot_AxesChart level.

Fix

Added an early-exit guard in Plot_AxesChart.paint():

if (width <= 0 || height <= 0) {
  this.bounds = new Rectangle2D.Double(xOffset, yOffset, 0, 0);
  return;
}

A zero-size Rectangle2D is still assigned so downstream callers like Legend_.paint() (which reads chart.getPlot().getBounds()) always receive a non-null value; Legend_'s own existing width < 30 check then correctly skips painting.

Testing

• Added paintingTinyChartDoesNotThrow() — paints a 1×1 chart
• Added paintingTinyChartWithMultipleYAxisGroupsDoesNotThrow() — paints a 1×1 chart with 3 Y-axis groups (the original failure scenario)
• Added TestForIssue712.java demo — run and resize the window as narrow as possible to verify no exception is thrown
• Full xchart test suite passes (mvn test -pl xchart)

[Copilot]

Pull request overview

Fixes #712 by preventing axes-based charts from attempting to paint when computed plot bounds become non-positive (e.g., extremely narrow XChartPanel with multiple Y-axis groups), which previously could trigger repeated NullPointerExceptions during rendering.

Changes:

• Add an early-exit guard in Plot_AxesChart.paint() when plot width <= 0 or height <= 0, while still assigning a non-null bounds rectangle.
• Add regression tests to ensure painting very small XY charts (including multiple Y-axis groups) does not throw.
• Add a standalone demo (TestForIssue712) to reproduce/validate the narrow-resize scenario interactively.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
xchart/src/main/java/org/knowm/xchart/internal/chartpart/Plot_AxesChart.java	Adds guard against zero/negative plot bounds to avoid downstream renderer failures.
xchart/src/test/java/org/knowm/xchart/XYChartTest.java	Adds regression coverage for painting 1×1 charts (single and multi Y-axis groups).
xchart-demo/src/main/java/org/knowm/xchart/standalone/issues/TestForIssue712.java	Adds a runnable demo to manually verify no exception when resizing extremely narrow.