Comprehensive logging system to eliminate unwanted console output (Issue #84) (#86)

* feat: Add comprehensive logging system to replace console output

- Add fortplot_logging module with configurable log levels (SILENT, ERROR, WARNING, INFO, DEBUG)
- Replace all print statements with appropriate log calls
- Export logging interface through main fortplot module for user control
- Default to WARNING level (errors and warnings only)
- Users can set LOG_LEVEL_SILENT for production use with zero console output

**Affected files:**
- fortplot.f90: Replace error messages for unimplemented features
- fortplot_png.f90: Replace compression failure and success messages
- fortplot_pdf.f90: Replace file creation success message
- fortplot_text.f90: Replace STB TrueType initialization errors
- fortplot_animation.f90: Replace all animation progress and error messages
- fortplot_gltf.f90: Replace GLTF/GLB file creation messages
- fortplot_figure_core.f90: Replace user interaction prompts and warnings
- fortplot_raster.f90: Replace backend usage error
- fortplot_ascii.f90: Replace file save confirmation message

**User Interface:**
- call set_log_level(LOG_LEVEL_SILENT) for production apps with zero output
- call set_log_level(LOG_LEVEL_INFO) for verbose operation
- call set_log_level(LOG_LEVEL_DEBUG) for maximum verbosity

Fixes #84

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

* fix: resolve critical CI failures blocking PR merges

- Fix line length violation in fortplot_figure_core.f90 (132 chars -> 88 char limit)
- Restore CMakeLists.txt required for CI cmake builds
- Autonomous fix during batch mode PR processing to enable merging

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

---------

Co-authored-by: Claude <noreply@anthropic.com>

Author: krystophny

src/fortplot.f90

@@ -18,6 +18,9 @@ module fortplot
     use fortplot_figure, only: figure_t
     use fortplot_format_parser, only: parse_format_string, contains_format_chars
     use fortplot_animation, only: animation_t, FuncAnimation
+    use fortplot_logging, only: set_log_level, log_error, log_warning, log_info, &
+                                LOG_LEVEL_SILENT, LOG_LEVEL_ERROR, &
+                                LOG_LEVEL_WARNING, LOG_LEVEL_INFO, LOG_LEVEL_DEBUG
 
     implicit none
 
@@ -37,6 +40,10 @@ module fortplot
 
     ! Animation interface
     public :: animation_t, FuncAnimation
+    
+    ! Logging interface
+    public :: set_log_level, LOG_LEVEL_SILENT, LOG_LEVEL_ERROR, &
+              LOG_LEVEL_WARNING, LOG_LEVEL_INFO, LOG_LEVEL_DEBUG
 
     ! Line style constants (pyplot-style)
     character(len=*), parameter, public :: LINESTYLE_SOLID = '-'
@@ -225,7 +232,7 @@ subroutine hist(data, bins, density, label, color)
         call ensure_global_figure_initialized()
         ! TODO: Implement hist method in figure_core
         ! call fig%hist(data, bins=bins, density=density, label=label, color=color)
-        print *, "ERROR: hist() not yet implemented - please use main branch for histogram support"
+        call log_error("hist() not yet implemented - please use main branch for histogram support")
     end subroutine hist
 
     subroutine histogram(data, bins, density, label, color)
@@ -253,7 +260,7 @@ subroutine histogram(data, bins, density, label, color)
         call ensure_global_figure_initialized()
         ! TODO: Implement hist method in figure_core
         ! call fig%hist(data, bins=bins, density=density, label=label, color=color)
-        print *, "ERROR: hist() not yet implemented - please use main branch for histogram support"
+        call log_error("hist() not yet implemented - please use main branch for histogram support")
     end subroutine histogram
 
     subroutine boxplot(data, position, width, label, show_outliers, horizontal, color)
@@ -285,7 +292,7 @@ subroutine boxplot(data, position, width, label, show_outliers, horizontal, colo
         ! TODO: Implement boxplot method in figure_core
         ! call fig%boxplot(data, position=position, width=width, label=label, &
         !                show_outliers=show_outliers, horizontal=horizontal, color=color)
-        print *, "ERROR: boxplot() not yet implemented - please use main branch for boxplot support"
+        call log_error("boxplot() not yet implemented - please use main branch for boxplot support")
     end subroutine boxplot
 
     subroutine show_data(x, y, label, title_text, xlabel_text, ylabel_text, blocking)
@@ -666,13 +673,13 @@ subroutine show_viewer_implementation(blocking)
         call execute_command_line(command, wait=.false., exitstat=stat)
 
         if (stat /= 0) then
-            print *, 'Warning: Failed to open plot viewer. Plot saved to: ', trim(temp_filename)
-            print *, 'Please open the file manually with your preferred PDF viewer.'
+            call log_warning('Failed to open plot viewer. Plot saved to: ' // trim(temp_filename))
+            call log_info('Please open the file manually with your preferred PDF viewer.')
         else
-            print *, 'Plot opened in default viewer. File: ', trim(temp_filename)
+            call log_info('Plot opened in default viewer. File: ' // trim(temp_filename))
 
             if (do_block) then
-                print *, 'Press Enter to continue and clean up temporary file...'
+                call log_info('Press Enter to continue and clean up temporary file...')
                 read(*,*)
 
                 ! Clean up temporary file
@@ -688,7 +695,7 @@ subroutine show_viewer_implementation(blocking)
                 call execute_command_line(command)
             else
                 ! In non-blocking mode, just inform that file stays
-                print *, 'Note: Temporary file will remain at: ', trim(temp_filename)
+            call log_info('Note: Temporary file will remain at: ' // trim(temp_filename))
             end if
         end if
     end subroutine show_viewer_implementation

src/fortplot_animation.f90

@@ -4,6 +4,7 @@ module fortplot_animation
     use fortplot_pipe, only: open_ffmpeg_pipe, write_png_to_pipe, close_ffmpeg_pipe
     use fortplot_png, only: png_context, create_png_canvas, get_png_data
     use fortplot_mpeg1_format, only: encode_animation_to_mpeg1
+    use fortplot_logging, only: log_error, log_info, log_warning
     implicit none
     private
 
@@ -63,11 +64,11 @@ subroutine run(self)
         integer :: i
 
         if (.not. associated(self%animate_func)) then
-            print *, "Error: Animation callback function not associated"
+            call log_error("Animation callback function not associated")
             return
         end if
 
-        print *, "Running animation with", self%frames, "frames..."
+        call log_info("Running animation with frames...")
 
         do i = 1, self%frames
             call self%animate_func(i)
@@ -78,7 +79,7 @@ subroutine run(self)
             end if
         end do
 
-        print *, "Animation completed."
+        call log_info("Animation completed.")
     end subroutine run
 
     subroutine set_save_frames(self, pattern)
@@ -119,13 +120,13 @@ subroutine save(self, filename, fps, status)
 
         if (.not. is_video_format(extension)) then
             if (present(status)) status = -3
-            print *, "Error: Unsupported file format. Use .mp4, .avi, or .mkv"
+            call log_error("Unsupported file format. Use .mp4, .avi, or .mkv")
             return
         end if
 
         if (.not. check_ffmpeg_available()) then
             if (present(status)) status = -1
-            print *, "Error: ffmpeg not found. Please install ffmpeg to save animations."
+            call log_error("ffmpeg not found. Please install ffmpeg to save animations.")
             return
         end if
 
@@ -230,7 +231,7 @@ subroutine save_animation_with_ffmpeg_pipe(anim, filename, fps, status)
         if (should_use_native_encoder(anim, filename)) then
             call save_animation_with_native_mpeg1(anim, filename, fps, status)
             if (status == 0) return  ! Native encoder succeeded
-            print *, "Native MPEG-1 encoder failed, falling back to FFmpeg"
+            call log_warning("Native MPEG-1 encoder failed, falling back to FFmpeg")
         end if
 
         ! Fall back to FFmpeg pipeline
@@ -327,23 +328,23 @@ subroutine save_animation_with_ffmpeg_pipeline(anim, filename, fps, status)
         stat = open_ffmpeg_pipe(filename, fps)
         if (stat /= 0) then
             status = -4
-            print *, "Error: Could not open pipe to ffmpeg"
+            call log_error("Could not open pipe to ffmpeg")
             return
         end if
 
         do frame_idx = 1, anim%frames
             call generate_png_frame_data(anim, frame_idx, png_data, stat)
             if (stat /= 0) then
                 status = -5
-                print *, "Error: Failed to generate frame", frame_idx
+                call log_error("Failed to generate frame")
                 stat = close_ffmpeg_pipe()
                 return
             end if
 
             stat = write_png_to_pipe(png_data)
             if (stat /= 0) then
                 status = -6
-                print *, "Error: Failed to write frame to pipe", frame_idx
+                call log_error("Failed to write frame to pipe")
                 stat = close_ffmpeg_pipe()
                 return
             end if
@@ -358,7 +359,7 @@ subroutine save_animation_with_ffmpeg_pipeline(anim, filename, fps, status)
             status = 0
         else
             status = -7
-            print *, "Error: Generated video failed validation"
+            call log_error("Generated video failed validation")
         end if
     end subroutine save_animation_with_ffmpeg_pipeline
 

src/fortplot_ascii.f90

@@ -8,6 +8,7 @@ module fortplot_ascii
     !! Author: fortplot contributors
 
     use fortplot_context
+    use fortplot_logging, only: log_info
     use fortplot_latex_parser
     use fortplot_unicode
     use, intrinsic :: iso_fortran_env, only: wp => real64
@@ -251,7 +252,7 @@ subroutine ascii_finalize(this, filename)
             open(newunit=unit, file=filename, status='replace')
             call output_to_file(this, unit)
             close(unit)
-            print *, "Unicode plot saved to '", trim(filename), "'"
+            call log_info("Unicode plot saved to '" // trim(filename) // "'")
         end if
     end subroutine ascii_finalize
 

src/fortplot_figure_core.f90

@@ -13,6 +13,7 @@ module fortplot_figure_core
     use fortplot_scales
     use fortplot_utils
     use fortplot_axes
+    use fortplot_logging, only: log_warning, log_info
     use fortplot_gltf, only: gltf_context
     use fortplot_colormap
     use fortplot_pcolormesh
@@ -631,11 +632,11 @@ subroutine savefig(self, filename, blocking)
             call self%backend%save(filename)
         end select
 
-        write(*, '(A, A, A)') 'Saved figure: ', trim(filename)
+        call log_info('Saved figure: ' // trim(filename))
 
         ! If blocking requested, wait for user input
         if (do_block) then
-            print *, "Press Enter to continue..."
+            call log_info("Press Enter to continue...")
             read(*,*)
         end if
     end subroutine savefig
@@ -664,7 +665,7 @@ subroutine show(self, blocking)
 
         ! If blocking requested, wait for user input
         if (do_block) then
-            print *, "Press Enter to continue..."
+            call log_info("Press Enter to continue...")
             read(*,*)
         end if
     end subroutine show
@@ -2537,23 +2538,22 @@ subroutine set_ydata(self, plot_index, y_new)
         real(wp), intent(in) :: y_new(:)
 
         if (plot_index < 1 .or. plot_index > self%plot_count) then
-            print *, "Warning: Invalid plot index", plot_index, "for set_ydata"
+            call log_warning("Invalid plot index for set_ydata")
             return
         end if
 
         if (self%plots(plot_index)%plot_type /= PLOT_TYPE_LINE) then
-            print *, "Warning: set_ydata only supported for line plots"
+            call log_warning("set_ydata only supported for line plots")
             return
         end if
 
         if (.not. allocated(self%plots(plot_index)%y)) then
-            print *, "Warning: Plot", plot_index, "has no y data to update"
+            call log_warning("Plot has no y data to update")
             return
         end if
 
         if (size(y_new) /= size(self%plots(plot_index)%y)) then
-            print *, "Warning: New y data size", size(y_new), &
-                     "does not match existing size", size(self%plots(plot_index)%y)
+            call log_warning("New y data size does not match existing size")
             return
         end if
 
@@ -2635,7 +2635,7 @@ subroutine ensure_directory_exists(filename)
             write(command, '(A,A,A)') 'mkdir -p "', trim(dir_path), '"'
             call execute_command_line(command, exitstat=status)
             if (status /= 0) then
-                print *, "Warning: Could not create directory: ", trim(dir_path)
+                call log_warning("Could not create directory: " // trim(dir_path))
             end if
         end if
     end subroutine ensure_directory_exists

src/fortplot_gltf.f90

@@ -6,6 +6,7 @@ module fortplot_gltf
     use fortplot_context, only: plot_context, setup_canvas
     use fortplot_gltf_base
     use fortplot_gltf_writer
+    use fortplot_logging, only: log_info, log_error
     use fortplot_gltf_geometry
     use fortplot_gltf_buffer
     use fortplot_glb_writer
@@ -76,9 +77,9 @@ subroutine save_gltf(this, filename)
             ! Write GLB binary format
             if (allocated(this%buffer_data)) then
                 call write_glb_file(filename, json, this%buffer_data)
-                print *, "GLB file '" // trim(filename) // "' created successfully!"
+                call log_info("GLB file '" // trim(filename) // "' created successfully!")
             else
-                print *, "Error: No binary data for GLB file"
+                call log_error("No binary data for GLB file")
             end if
         else
             ! Write GLTF text format
@@ -87,9 +88,9 @@ subroutine save_gltf(this, filename)
             if (iostat == 0) then
                 write(unit, '(A)') json
                 close(unit)
-                print *, "GLTF file '" // trim(filename) // "' created successfully!"
+                call log_info("GLTF file '" // trim(filename) // "' created successfully!")
             else
-                print *, "Error: Failed to create GLTF file"
+                call log_error("Failed to create GLTF file")
             end if
         end if
 

src/fortplot_logging.f90

@@ -0,0 +1,65 @@
+module fortplot_logging
+    !! Simple logging facility for fortplot library
+    !! Allows control over console output verbosity
+    
+    implicit none
+    private
+    
+    public :: set_log_level, log_info, log_warning, log_error, log_debug
+    public :: LOG_LEVEL_SILENT, LOG_LEVEL_ERROR, LOG_LEVEL_WARNING, LOG_LEVEL_INFO, LOG_LEVEL_DEBUG
+    
+    ! Log levels (in increasing verbosity)
+    integer, parameter :: LOG_LEVEL_SILENT = 0
+    integer, parameter :: LOG_LEVEL_ERROR = 1  
+    integer, parameter :: LOG_LEVEL_WARNING = 2
+    integer, parameter :: LOG_LEVEL_INFO = 3
+    integer, parameter :: LOG_LEVEL_DEBUG = 4
+    
+    ! Default log level (warnings and errors only)
+    integer :: current_log_level = LOG_LEVEL_WARNING
+    
+contains
+
+    subroutine set_log_level(level)
+        !! Set the global logging level
+        !! 
+        !! Arguments:
+        !!   level: LOG_LEVEL_SILENT, LOG_LEVEL_ERROR, LOG_LEVEL_WARNING, 
+        !!          LOG_LEVEL_INFO, or LOG_LEVEL_DEBUG
+        integer, intent(in) :: level
+        current_log_level = level
+    end subroutine set_log_level
+
+    subroutine log_info(message)
+        !! Log an informational message
+        character(len=*), intent(in) :: message
+        if (current_log_level >= LOG_LEVEL_INFO) then
+            print *, "[INFO] ", trim(message)
+        end if
+    end subroutine log_info
+
+    subroutine log_warning(message)
+        !! Log a warning message
+        character(len=*), intent(in) :: message
+        if (current_log_level >= LOG_LEVEL_WARNING) then
+            print *, "[WARNING] ", trim(message)
+        end if
+    end subroutine log_warning
+
+    subroutine log_error(message)
+        !! Log an error message
+        character(len=*), intent(in) :: message
+        if (current_log_level >= LOG_LEVEL_ERROR) then
+            print *, "[ERROR] ", trim(message)
+        end if
+    end subroutine log_error
+
+    subroutine log_debug(message)
+        !! Log a debug message
+        character(len=*), intent(in) :: message
+        if (current_log_level >= LOG_LEVEL_DEBUG) then
+            print *, "[DEBUG] ", trim(message)
+        end if
+    end subroutine log_debug
+
+end module fortplot_logging

src/fortplot_pdf.f90

@@ -3,6 +3,7 @@ module fortplot_pdf
     use fortplot_vector, only: vector_stream_writer, vector_graphics_state
     use fortplot_latex_parser
     use fortplot_unicode
+    use fortplot_logging, only: log_info
     use fortplot_margins, only: plot_margins_t, plot_area_t, calculate_plot_area, get_axis_tick_positions
     use fortplot_ticks, only: generate_scale_aware_tick_labels, find_nice_tick_locations, format_tick_value_smart
     use fortplot_label_positioning, only: calculate_x_label_position, calculate_y_label_position, &
@@ -427,7 +428,7 @@ subroutine write_pdf_file(this, filename)
 
         call finalize_pdf_stream(this)
         call create_pdf_document(unit, filename, this)
-        print *, "PDF file '", trim(filename), "' created successfully!"
+        call log_info("PDF file '" // trim(filename) // "' created successfully!")
     end subroutine write_pdf_file
 
     subroutine normalize_to_pdf_coords(ctx, x, y, pdf_x, pdf_y)

src/fortplot_png.f90

@@ -3,6 +3,7 @@ module fortplot_png
     use fortplot_context, only: setup_canvas
     use fortplot_raster, only: raster_context, create_raster_canvas, draw_axes_and_labels, draw_rotated_ylabel_raster
     use fortplot_zlib, only: zlib_compress, crc32_calculate
+    use fortplot_logging, only: log_error, log_info
     use, intrinsic :: iso_fortran_env, only: wp => real64, int8, int32
     implicit none
 
@@ -57,7 +58,7 @@ subroutine generate_png_data(width, height, image_data, png_buffer)
         compressed_data = zlib_compress(image_data, data_size, compressed_size)
 
         if (.not. allocated(compressed_data) .or. compressed_size <= 0) then
-            print *, "Compression failed"
+            call log_error("PNG compression failed")
             return
         end if
 
@@ -124,7 +125,7 @@ subroutine write_png_file(filename, width, height, image_data)
         close(png_unit)
 
         deallocate(png_buffer)
-        print *, "PNG file '", trim(filename), "' created successfully!"
+        call log_info("PNG file '" // trim(filename) // "' created successfully!")
     end subroutine write_png_file
 
     ! Public wrapper for getting PNG data