refactor: extract label positioning constants to shared module

src/fortplot.f90

@@ -55,7 +55,9 @@ module fortplot
 
     ! Core types and constants
     use fortplot_figure_core, only: figure_t
-    use fortplot_constants, only: EPSILON_COMPARE, EPSILON_GEOMETRY
+    use fortplot_constants, only: EPSILON_COMPARE, EPSILON_GEOMETRY, &
+                                  XLABEL_VERTICAL_OFFSET, YLABEL_HORIZONTAL_OFFSET, &
+                                  TICK_MARK_LENGTH, TITLE_VERTICAL_OFFSET
 
     ! Animation functionality
     use fortplot_animation, only: animation_t, FuncAnimation
@@ -104,7 +106,9 @@ module fortplot
     public :: figure_t, wp
 
     ! Numerical constants
-    public :: EPSILON_COMPARE, EPSILON_GEOMETRY
+    public :: EPSILON_COMPARE, EPSILON_GEOMETRY, &
+              XLABEL_VERTICAL_OFFSET, YLABEL_HORIZONTAL_OFFSET, TICK_MARK_LENGTH, &
+              TITLE_VERTICAL_OFFSET
 
     ! Coordinate system constants for annotations
     public :: COORD_DATA, COORD_FIGURE, COORD_AXIS

src/fortplot_constants.f90

@@ -43,4 +43,44 @@ module fortplot_constants
     !! Smaller value provides better precision for critical geometric operations.
     real(wp), parameter, public :: EPSILON_GEOMETRY = 1e-12_wp
 
+    ! ========================================================================
+    ! LABEL POSITIONING CONSTANTS
+    ! ========================================================================
+
+    !! X-axis label vertical offset below plot area (pixels)
+    !!
+    !! This constant defines the vertical distance between the bottom of the
+    !! plot area and the baseline of the x-axis label text.
+    !! 
+    !! Used by raster backend for xlabel positioning to ensure consistent
+    !! spacing below tick labels while avoiding overlap.
+    integer, parameter, public :: XLABEL_VERTICAL_OFFSET = 30
+
+    !! Y-axis label horizontal offset from left edge (pixels)
+    !!
+    !! This constant defines the horizontal distance from the left edge of
+    !! the figure to the left edge of the y-axis label text.
+    !!
+    !! Used by raster backend for ylabel positioning to ensure consistent
+    !! left margin spacing and readability.
+    integer, parameter, public :: YLABEL_HORIZONTAL_OFFSET = 10
+
+    !! Tick mark length extending from axis line (pixels)
+    !!
+    !! This constant defines the length of tick marks extending from the
+    !! axis lines into the plot area or margin.
+    !!
+    !! Used by raster backend for drawing consistent tick marks across
+    !! all axes while maintaining visual hierarchy.
+    integer, parameter, public :: TICK_MARK_LENGTH = 5
+
+    !! Title vertical offset above plot area (pixels)
+    !!
+    !! This constant defines the vertical distance between the bottom of the
+    !! plot area and the baseline of the title text.
+    !!
+    !! Used by raster backend for title positioning to ensure consistent
+    !! spacing above the plot area while maintaining readability.
+    integer, parameter, public :: TITLE_VERTICAL_OFFSET = 30
+
 end module fortplot_constants

src/fortplot_raster.f90

@@ -1,7 +1,9 @@
 module fortplot_raster
     use iso_c_binding
     use fortplot_context, only: plot_context, setup_canvas
-    use fortplot_constants, only: EPSILON_COMPARE, EPSILON_GEOMETRY
+    use fortplot_constants, only: EPSILON_COMPARE, EPSILON_GEOMETRY, &
+                                  XLABEL_VERTICAL_OFFSET, YLABEL_HORIZONTAL_OFFSET, &
+                                  TICK_MARK_LENGTH, TITLE_VERTICAL_OFFSET
     use fortplot_text, only: render_text_to_image, calculate_text_width, calculate_text_height
     use fortplot_latex_parser, only: process_latex_in_text
     ! use fortplot_unicode, only: unicode_to_ascii
@@ -983,7 +985,7 @@ subroutine raster_draw_axes_and_labels(this, xscale, yscale, symlog_threshold, &
         call this%line(x_min, y_min, x_min, y_max)
 
         ! Generate and draw tick marks and labels
-        tick_length = 5  ! Tick length in pixels
+        tick_length = TICK_MARK_LENGTH  ! Tick length in pixels
 
         ! X-axis ticks
         call compute_scale_ticks(xscale, x_min, x_max, symlog_threshold, x_tick_positions, num_x_ticks)
@@ -1055,7 +1057,7 @@ subroutine raster_draw_axes_and_labels(this, xscale, yscale, symlog_threshold, &
                 text_width = calculate_text_width(trim(escaped_text))
                 ! Center horizontally in plot area, position below tick labels
                 px = this%plot_area%left + this%plot_area%width / 2 - text_width / 2
-                py = this%plot_area%bottom + this%plot_area%height + 30  ! 30 pixels below plot area
+                py = this%plot_area%bottom + this%plot_area%height + XLABEL_VERTICAL_OFFSET  ! Pixels below plot area
                 call render_text_to_image(this%raster%image_data, this%width, this%height, &
                                         px, py, trim(escaped_text), text_r, text_g, text_b)
             end if
@@ -1069,7 +1071,7 @@ subroutine raster_draw_axes_and_labels(this, xscale, yscale, symlog_threshold, &
                 text_width = calculate_text_width(trim(escaped_text))
                 text_height = calculate_text_height(trim(escaped_text))
                 ! Position to the left of plot area, centered vertically
-                px = 10  ! 10 pixels from left edge
+                px = YLABEL_HORIZONTAL_OFFSET  ! Pixels from left edge
                 py = this%plot_area%bottom + this%plot_area%height / 2 - text_height / 2
                 call render_text_to_image(this%raster%image_data, this%width, this%height, &
                                         px, py, trim(escaped_text), text_r, text_g, text_b)
@@ -1128,7 +1130,7 @@ subroutine render_title_centered(this, title_text)
 
         ! Y position: above plot area (like matplotlib)  
         ! Place title approximately 30 pixels above the plot area
-        title_py = real(this%plot_area%bottom - 30, wp)
+        title_py = real(this%plot_area%bottom - TITLE_VERTICAL_OFFSET, wp)
 
         ! Get current color and render title directly in pixel coordinates
         call this%raster%get_color_bytes(r, g, b)