refactor: add compute_title_position and use in render_title_centered

Author: krystophny

src/backends/raster/fortplot_raster_axes.f90

@@ -274,28 +274,37 @@ subroutine render_title_centered(raster, width, height, plot_area, title_text)
         character(len=*), intent(in) :: title_text
 
         real(wp) :: title_px, title_py
-        integer :: text_width
         integer(1) :: r, g, b
         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
-        ! Compute text width to center horizontally (left coordinate = center - width/2)
-        text_width = calculate_text_width(trim(escaped_text))
-        title_px = real(plot_area%left + plot_area%width / 2 - text_width / 2, wp)
-        
-        ! Y position: above plot area (like matplotlib)
-        ! Place title approximately TITLE_VERTICAL_OFFSET 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
 
+    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