fix: center PNG title over plot area (fixes #999)

- CI: All checks green post-rebase (Linux tests, coverage, Windows)\n- Local: make test-ci and make test passed\n- Review: small, focused change; adds compute_title_position and a raster title centering test; no backward-incompatible changes

Author: krystophny

src/backends/raster/fortplot_raster_axes.f90

@@ -14,6 +14,7 @@ module fortplot_raster_axes
 
     private
     public :: raster_draw_axes_and_labels, raster_render_ylabel
+    public :: compute_title_position
 
 contains
 
@@ -277,22 +278,33 @@ subroutine render_title_centered(raster, width, height, plot_area, title_text)
         character(len=500) :: processed_text, escaped_text
         integer :: processed_len
 
-        ! Process LaTeX commands and Unicode
-        call process_latex_in_text(title_text, processed_text, processed_len)
-        call escape_unicode_for_raster(processed_text(1:processed_len), escaped_text)
-        
-        ! Calculate title position centered over plot area
-        ! X position: center of plot area horizontally
-        title_px = real(plot_area%left + plot_area%width / 2, wp)
-        
-        ! Y position: above plot area (like matplotlib)  
-        ! Place title approximately 30 pixels above the plot area
-        title_py = real(plot_area%bottom - TITLE_VERTICAL_OFFSET, wp)
+        ! Compute title position (centered like matplotlib)
+        call compute_title_position(plot_area, title_text, processed_text, processed_len, escaped_text, title_px, title_py)
 
         ! Get current color and render title directly in pixel coordinates
         call raster%get_color_bytes(r, g, b)
         call render_text_to_image(raster%image_data, width, height, &
                                  int(title_px), int(title_py), trim(escaped_text), r, g, b)
     end subroutine render_title_centered
 
-end module fortplot_raster_axes
+    subroutine compute_title_position(plot_area, title_text, processed_text, processed_len, escaped_text, title_px, title_py)
+        !! Compute centered title position and return processed/escaped text
+        !! Exposed for reuse and testing; keeps render routine small and focused
+        type(plot_area_t), intent(in) :: plot_area
+        character(len=*), intent(in) :: title_text
+        character(len=*), intent(out) :: processed_text
+        integer, intent(out) :: processed_len
+        character(len=*), intent(out) :: escaped_text
+        real(wp), intent(out) :: title_px, title_py
+
+        integer :: text_width
+
+        call process_latex_in_text(title_text, processed_text, processed_len)
+        call escape_unicode_for_raster(processed_text(1:processed_len), escaped_text)
+
+        text_width = calculate_text_width(trim(escaped_text))
+        title_px = real(plot_area%left + plot_area%width / 2 - text_width / 2, wp)
+        title_py = real(plot_area%bottom - TITLE_VERTICAL_OFFSET, wp)
+    end subroutine compute_title_position
+
+end module fortplot_raster_axes

test/test_title_centering_raster.f90

@@ -0,0 +1,43 @@
+program test_title_centering_raster
+    !! Verifies raster title centering over plot area
+    use fortplot_layout, only: plot_margins_t, plot_area_t, calculate_plot_area
+    use fortplot_constants, only: TITLE_VERTICAL_OFFSET
+    use fortplot_text, only: calculate_text_width
+    use fortplot_raster_axes, only: compute_title_position
+    use, intrinsic :: iso_fortran_env, only: wp => real64
+    implicit none
+
+    type(plot_margins_t) :: margins
+    type(plot_area_t) :: plot_area
+    integer, parameter :: CANVAS_WIDTH = 640
+    integer, parameter :: CANVAS_HEIGHT = 480
+
+    character(len=*), parameter :: title_text = 'Simple Sine Wave'
+    character(len=500) :: processed_text, escaped_text
+    integer :: processed_len
+    real(wp) :: title_px_r, title_py_r
+    integer :: expected_px, expected_py, measured_width
+
+    margins = plot_margins_t()
+    call calculate_plot_area(CANVAS_WIDTH, CANVAS_HEIGHT, margins, plot_area)
+
+    call compute_title_position(plot_area, title_text, processed_text, processed_len, &
+                                escaped_text, title_px_r, title_py_r)
+
+    measured_width = calculate_text_width(trim(escaped_text))
+    expected_px = plot_area%left + plot_area%width/2 - measured_width/2
+    expected_py = plot_area%bottom - TITLE_VERTICAL_OFFSET
+
+    if (int(title_px_r) /= expected_px) then
+        print *, 'FAIL: Title X not centered; got ', int(title_px_r), ' expected ', expected_px
+        stop 1
+    end if
+
+    if (int(title_py_r) /= expected_py) then
+        print *, 'FAIL: Title Y offset incorrect; got ', int(title_py_r), ' expected ', expected_py
+        stop 1
+    end if
+
+    print *, 'PASS: Title centering matches expected pixel position'
+end program test_title_centering_raster
+