fix: reset plot_count unconditionally during figure initialization

[krystophny]

Summary

• Fixes Issue Old plot not cleared in second figure ... figure() call? https://lazy-fortran.github.io/fortplot/page/examples/contour_demo.html #330: "Fix - Need to check figure clearing - contour_demo.html has overlapping contours"
• Root cause: plot_count was only reset when first allocating the plots array, causing subsequent figure() calls to accumulate plots instead of clearing them
• Solution: Move plot_count = 0 outside the allocation check to reset unconditionally during every initialize() call

Changes

• Fixed initialize() method in src/fortplot_figure_core.f90 to reset plot_count unconditionally
• Added test case test/test_figure_clear_330.f90 to validate proper figure clearing behavior
• Preserves allocation logic - plots array is still only allocated once for efficiency

Test Plan

• Created focused test case for figure clearing functionality
• Verified test passes with fix applied
• Ran contour_demo to confirm no more overlapping contours
• Ran related figure tests to ensure no regressions
• All tests pass successfully

Technical Details

Before Fix:

if (.not. allocated(self%plots)) then
    allocate(self%plots(self%max_plots))
    self%plot_count = 0  ! Only reset when allocating
end if

After Fix:

if (.not. allocated(self%plots)) then
    allocate(self%plots(self%max_plots))
end if
self%plot_count = 0  ! Reset on every figure() call

Expected Behavior After Fix:

1. First figure() call → allocates plots array and sets plot_count = 0
2. Add plots → plot_count increases normally
3. Second figure() call → resets plot_count = 0 (plots array already allocated)
4. Add new plots → start from index 0, replacing previous plots

This ensures proper plot isolation between figure() calls and resolves the overlapping contours issue.

fixes #330

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/fortplot_figure_core.f90	0.00%	1 Missing ⚠️

📢 Thoughts on this report? Let us know!