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fix: PNG backend twin axes top-axis tick label overlaps plot title (fixes #1700) #1822

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Merged
krystophny merged 1 commit into from
Apr 26, 2026
Merged

fix: PNG backend twin axes top-axis tick label overlaps plot title (fixes #1700) #1822

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63 changes: 46 additions & 17 deletions src/backends/raster/fortplot_raster_labels.f90
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Original file line number Diff line number Diff line change
Expand Up @@ -5,11 +5,11 @@ module fortplot_raster_labels
YLABEL_EXTRA_GAP, TITLE_VERTICAL_OFFSET, &
REFERENCE_DPI, FALLBACK_LABEL_HEIGHT_PX, &
MIN_LABEL_MARGIN_PX, CANVAS_EDGE_PADDING_PX
use fortplot_text_rendering, only: render_text_to_image, calculate_text_width, &
calculate_text_height, &
calculate_text_descent, &
calculate_text_width_with_size, &
render_text_with_size, TITLE_FONT_SIZE
use fortplot_text_rendering, only: render_text_to_image, calculate_text_width, &
calculate_text_height, &
calculate_text_descent, &
calculate_text_width_with_size, &
render_text_with_size, TITLE_FONT_SIZE
use fortplot_text_fonts, only: get_font_ascent_ratio
use fortplot_text_helpers, only: prepare_text_for_raster
use fortplot_margins, only: plot_area_t
Expand Down Expand Up @@ -316,8 +316,8 @@ subroutine raster_draw_top_xlabel(raster, width, height, plot_area, xlabel)
trim(escaped_text), 0_1, 0_1, 0_1)
end subroutine raster_draw_top_xlabel

subroutine render_title_centered(raster, width, height, plot_area, &
title_text, custom_font_size)
subroutine render_title_centered(raster, width, height, plot_area, &
title_text, custom_font_size)
!! Render title centered above the plot area
type(raster_image_t), intent(inout) :: raster
integer, intent(in) :: width, height
Expand All @@ -337,7 +337,7 @@ subroutine render_title_centered(raster, width, height, plot_area, &

call compute_title_position_sized(plot_area, title_text, &
escaped_text, title_px_real, title_py_real, &
fsize, raster%dpi)
fsize, raster%dpi, raster%last_x_tick_max_height_top)

title_px = int(title_px_real)
title_py = int(title_py_real)
Expand All @@ -348,40 +348,69 @@ subroutine render_title_centered(raster, width, height, plot_area, &
fsize)
end subroutine render_title_centered

subroutine compute_title_position(plot_area, title_text, escaped_text, &
title_px, title_py, dpi)
subroutine compute_title_position(plot_area, title_text, escaped_text, &
title_px, title_py, dpi, &
top_tick_height)
!! Compute the position for centered title above plot area
type(plot_area_t), intent(in) :: plot_area
character(len=*), intent(in) :: title_text
character(len=*), intent(out) :: escaped_text
real(wp), intent(out) :: title_px, title_py
real(wp), intent(in), optional :: dpi
integer, intent(in), optional :: top_tick_height
integer :: tick_h

tick_h = -1
if (present(top_tick_height)) tick_h = top_tick_height

call compute_title_position_sized(plot_area, title_text, &
escaped_text, title_px, title_py, real(TITLE_FONT_SIZE, wp), dpi)
escaped_text, title_px, title_py, real(TITLE_FONT_SIZE, wp), &
dpi, tick_h)
end subroutine compute_title_position

subroutine compute_title_position_sized(plot_area, title_text, &
escaped_text, title_px, &
title_py, fsize, dpi)
subroutine compute_title_position_sized(plot_area, title_text, &
escaped_text, title_px, &
title_py, fsize, dpi, &
top_tick_height)
!! Compute centered title position with explicit font size.
!! When top_tick_height > 0 (twiny active), the title is placed
!! above the top-axis xlabel to avoid overlap with tick labels
!! and the top x-axis label.
type(plot_area_t), intent(in) :: plot_area
character(len=*), intent(in) :: title_text
character(len=*), intent(out) :: escaped_text
real(wp), intent(out) :: title_px, title_py
real(wp), intent(in) :: fsize
real(wp), intent(in), optional :: dpi
integer :: title_width
integer, intent(in) :: top_tick_height
integer :: title_width, title_height
real(wp) :: dpi_val
integer :: top_xlabel_y, title_gap

dpi_val = REFERENCE_DPI
if (present(dpi)) dpi_val = dpi

call prepare_text_for_raster(title_text, escaped_text)

title_width = calculate_text_width_with_size( &
trim(escaped_text), fsize)
title_height = max(1, int(fsize))

title_px = real(plot_area%left + plot_area%width / 2 &
- title_width / 2, wp)
title_py = real(plot_area%bottom - TITLE_VERTICAL_OFFSET, wp)
title_py = max(1.0_wp, title_py)

if (top_tick_height > 0) then
! twiny active: place title above the top xlabel
top_xlabel_y = plot_area%bottom - &
scale_px(X_TICK_LABEL_TOP_PAD, dpi_val) - &
top_tick_height - title_height - &
CANVAS_EDGE_PADDING_PX
title_gap = TITLE_VERTICAL_OFFSET
title_py = real(max(1, top_xlabel_y - title_gap - title_height), wp)
else
title_py = real(plot_area%bottom - TITLE_VERTICAL_OFFSET, wp)
title_py = max(1.0_wp, title_py)
end if
end subroutine compute_title_position_sized

subroutine render_title_centered_with_size(raster, width, height, center_x, &
Expand Down
41 changes: 41 additions & 0 deletions test/test_title_position_twiny.f90
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Original file line number Diff line number Diff line change
@@ -0,0 +1,41 @@
program test_title_position_twiny
!! Test that title positioning accounts for twiny top-axis elements
!! Regression test for issue #1700: PNG backend twin axes tick label overlap
use, intrinsic :: iso_fortran_env, only: wp => real64
use fortplot_layout, only: plot_margins_t, plot_area_t, calculate_plot_area
use fortplot_constants, only: TITLE_VERTICAL_OFFSET, REFERENCE_DPI
use fortplot_raster_axes, only: compute_title_position

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=600) :: escaped
real(wp) :: title_px, baseline_py, twiny_py
integer :: top_tick_h

margins = plot_margins_t()
call calculate_plot_area(CANVAS_WIDTH, CANVAS_HEIGHT, margins, plot_area)

! Test 1: without twiny
call compute_title_position(plot_area, 'Test Title', escaped, &
title_px, baseline_py)

! Test 2: with twiny (top_tick_height > 0)
top_tick_h = 14
call compute_title_position(plot_area, 'Test Title', escaped, &
title_px, twiny_py, &
top_tick_height=top_tick_h)

! Verify twiny title is above baseline
if (twiny_py >= baseline_py) then
print *, 'FAIL: Twiny title not above baseline:', &
twiny_py, '>=', baseline_py
stop 1
end if

print *, 'PASS: Title positioned correctly with and without twiny'
end program test_title_position_twiny
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