Fix #834: custom tick formatter breaks logarithmic axis

[timmolter]

Problem

When a custom axis tick label formatter was set alongside a logarithmic axis (setYAxisLogarithmic(true) + setCustomYAxisTickLabelsFormatter(...)), the logarithmic scaling was silently discarded. The formatter received linearly-spaced values (0, 1e8, 2e8 ...) instead of the expected powers-of-ten (1, 10, 100 ... 1e8), and the chart rendered as a linear axis.

The root cause: in both Axis_Y.getAxisTickCalculatorForY() and Axis_X.getAxisTickCalculatorForX(), the custom-formatter check came first in the if/else chain and unconditionally routed to AxisTickCalculator_Callback, which inherits the base linear calculate() method. The logarithmic branch was never reached.

Approach

The fix combines logarithmic tick-position calculation with custom label formatting:

• AxisTickCalculator_Logarithmic -- all formatterLogNumber.format() calls inside calculate() are replaced with axisFormat.format(). Since axisFormat defaults to formatterLogNumber, existing behavior is unchanged. Two new constructors accept a Function<Double, String> and set axisFormat = new Formatter_Custom(fn) before calculate() runs, so tick positions remain logarithmic while tick labels come from the callback.

• Axis_Y / Axis_X -- a new combined-case branch is added above the plain callback branch: when both a custom formatter and logarithmic mode are active, it routes to the new constructors instead of AxisTickCalculator_Callback.

No changes to base classes; the fix is fully contained in these three files.

Testing

• Two new unit tests in XYChartTest (Y-axis and X-axis variants) verify the chart renders without throwing.
• TestForIssue834.java demo reproduces the reporter's exact use case -- logarithmic Y-axis with an energy unit formatter (nJ / µJ / mJ / J). The axis now shows correct log spacing with custom labels.

[Copilot]

Pull request overview

Fixes #834 where enabling a custom axis tick label formatter caused logarithmic axes to silently fall back to linear scaling by ensuring logarithmic tick positions are still computed while allowing custom tick labels.

Changes:

• Extend AxisTickCalculator_Logarithmic to support custom label formatting while preserving logarithmic tick calculation.
• Update Axis_Y / Axis_X tick-calculator selection to route the “logarithmic + custom formatter” combination to the logarithmic calculator instead of the callback (linear) calculator.
• Add regression tests and a standalone demo reproducer for issue #834.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
xchart/src/test/java/org/knowm/xchart/XYChartTest.java	Adds regression tests for custom formatter + log axis combinations.
xchart/src/main/java/org/knowm/xchart/internal/chartpart/AxisTickCalculator_Logarithmic.java	Allows log tick calculator to use a custom formatter for labels while keeping log spacing.
xchart/src/main/java/org/knowm/xchart/internal/chartpart/Axis_Y.java	Routes Y-axis “custom formatter + log mode” to the log tick calculator.
xchart/src/main/java/org/knowm/xchart/internal/chartpart/Axis_X.java	Routes X-axis “custom formatter + log mode” to the log tick calculator.
xchart-demo/src/main/java/org/knowm/xchart/standalone/issues/TestForIssue834.java	Adds a runnable demo reproducing #834’s scenario.

[timmolter]