Fix #47: Support NaN values for disconnected line segments

example/disconnected_lines.f90

@@ -0,0 +1,63 @@
+program disconnected_lines
+    use fortplot
+    use fortplot_figure, only: figure_t
+    use, intrinsic :: ieee_arithmetic, only: ieee_value, ieee_quiet_nan
+    implicit none
+
+    type(figure_t) :: fig
+    real(8) :: x(11), y(11), nan
+    integer :: i
+
+    ! Get NaN value
+    nan = ieee_value(nan, ieee_quiet_nan)
+
+    call fig%initialize(800, 600)
+
+    ! Create data with three disconnected segments using NaN as separator
+    ! First segment: sine wave from 0 to pi
+    x(1:4) = [0.0_8, 1.047_8, 2.094_8, 3.142_8]  ! 0, pi/3, 2pi/3, pi
+    do i = 1, 4
+        y(i) = sin(x(i))
+    end do
+
+    ! NaN separator
+    x(5) = nan
+    y(5) = nan
+
+    ! Second segment: cosine wave from pi to 2pi
+    x(6:9) = [3.142_8, 4.189_8, 5.236_8, 6.283_8]  ! pi, 4pi/3, 5pi/3, 2pi
+    do i = 6, 9
+        y(i) = cos(x(i))
+    end do
+
+    ! NaN separator
+    x(10) = nan
+    y(10) = nan
+
+    ! Third segment: horizontal line at y=0.5
+    x(11) = 7.0_8
+    y(11) = 0.5_8
+
+    ! Plot disconnected segments with markers and lines
+    call fig%add_plot(x, y, label='Disconnected segments', linestyle='o-')
+
+    ! Add single point (will be disconnected from the line)
+    call fig%add_plot([8.0_8], [-0.5_8], linestyle='rs', label='Single point')
+
+    ! Configure plot
+    call fig%set_title('Disconnected Line Segments Example')
+    call fig%set_xlabel('x')
+    call fig%set_ylabel('y')
+    call fig%legend()
+
+    ! Save in multiple formats
+    call fig%savefig('build/example/disconnected_lines.png')
+    call fig%savefig('build/example/disconnected_lines.pdf')
+    call fig%savefig('build/example/disconnected_lines.txt')
+
+    print *, "Disconnected lines example saved to:"
+    print *, "  - build/example/disconnected_lines.png"
+    print *, "  - build/example/disconnected_lines.pdf"
+    print *, "  - build/example/disconnected_lines.txt"
+
+end program disconnected_lines

src/fortplot_figure_core.f90

@@ -1028,7 +1028,8 @@ subroutine render_line_plot(self, plot_idx)
     end subroutine render_line_plot
 
     subroutine render_markers(self, plot_idx)
-        !! Render markers at each data point
+        !! Render markers at each data point, skipping NaN values
+        use, intrinsic :: ieee_arithmetic, only: ieee_is_nan
         class(figure_t), intent(inout) :: self
         integer, intent(in) :: plot_idx
         character(len=:), allocatable :: marker
@@ -1042,6 +1043,9 @@ subroutine render_markers(self, plot_idx)
         if (marker == 'None') return
 
         do i = 1, size(self%plots(plot_idx)%x)
+            ! Skip points with NaN values
+            if (ieee_is_nan(self%plots(plot_idx)%x(i)) .or. ieee_is_nan(self%plots(plot_idx)%y(i))) cycle
+
             x_trans = apply_scale_transform(self%plots(plot_idx)%x(i), self%xscale, self%symlog_threshold)
             y_trans = apply_scale_transform(self%plots(plot_idx)%y(i), self%yscale, self%symlog_threshold)
             call self%backend%draw_marker(x_trans, y_trans, marker)
@@ -1411,13 +1415,20 @@ subroutine draw_line_with_style(self, plot_idx, linestyle)
     end subroutine draw_line_with_style
 
     subroutine render_solid_line(self, plot_idx)
-        !! Render solid line by drawing all segments
+        !! Render solid line by drawing all segments, breaking on NaN values
+        use, intrinsic :: ieee_arithmetic, only: ieee_is_nan
         class(figure_t), intent(inout) :: self
         integer, intent(in) :: plot_idx
         integer :: i
         real(wp) :: x1_screen, y1_screen, x2_screen, y2_screen
 
         do i = 1, size(self%plots(plot_idx)%x) - 1
+            ! Skip segment if either point contains NaN
+            if (ieee_is_nan(self%plots(plot_idx)%x(i)) .or. ieee_is_nan(self%plots(plot_idx)%y(i)) .or. &
+                ieee_is_nan(self%plots(plot_idx)%x(i+1)) .or. ieee_is_nan(self%plots(plot_idx)%y(i+1))) then
+                cycle
+            end if
+
             ! Apply scale transformations
             x1_screen = apply_scale_transform(self%plots(plot_idx)%x(i), self%xscale, self%symlog_threshold)
             y1_screen = apply_scale_transform(self%plots(plot_idx)%y(i), self%yscale, self%symlog_threshold)
@@ -1430,6 +1441,7 @@ end subroutine render_solid_line
 
     subroutine render_patterned_line(self, plot_idx, linestyle)
         !! Render line with continuous pattern across segments (matplotlib-style)
+        use, intrinsic :: ieee_arithmetic, only: ieee_is_nan
         class(figure_t), intent(inout) :: self
         integer, intent(in) :: plot_idx
         character(len=*), intent(in) :: linestyle
@@ -1439,25 +1451,41 @@ subroutine render_patterned_line(self, plot_idx, linestyle)
         real(wp) :: pattern(20), pattern_length
         integer :: pattern_size, pattern_index
         logical :: drawing
-        integer :: i
+        integer :: i, valid_count
         real(wp) :: x1_screen, y1_screen, x2_screen, y2_screen, dx, dy
 
         ! Get transformed data range for proper pattern scaling
         real(wp) :: x_range, y_range, plot_scale
         real(wp), allocatable :: x_trans(:), y_trans(:)
+        logical, allocatable :: valid_points(:)
 
         ! Transform all data points to get proper scaling
         allocate(x_trans(size(self%plots(plot_idx)%x)))
         allocate(y_trans(size(self%plots(plot_idx)%y)))
+        allocate(valid_points(size(self%plots(plot_idx)%x)))
 
+        valid_count = 0
         do i = 1, size(self%plots(plot_idx)%x)
-            x_trans(i) = apply_scale_transform(self%plots(plot_idx)%x(i), self%xscale, self%symlog_threshold)
-            y_trans(i) = apply_scale_transform(self%plots(plot_idx)%y(i), self%yscale, self%symlog_threshold)
+            valid_points(i) = .not. (ieee_is_nan(self%plots(plot_idx)%x(i)) .or. ieee_is_nan(self%plots(plot_idx)%y(i)))
+            if (valid_points(i)) then
+                x_trans(i) = apply_scale_transform(self%plots(plot_idx)%x(i), self%xscale, self%symlog_threshold)
+                y_trans(i) = apply_scale_transform(self%plots(plot_idx)%y(i), self%yscale, self%symlog_threshold)
+                valid_count = valid_count + 1
+            else
+                x_trans(i) = 0.0_wp
+                y_trans(i) = 0.0_wp
+            end if
         end do
 
-        x_range = maxval(x_trans) - minval(x_trans)
-        y_range = maxval(y_trans) - minval(y_trans)
-        plot_scale = max(x_range, y_range)
+        ! Handle case where all points are NaN
+        if (valid_count > 0) then
+            x_range = maxval(x_trans, mask=valid_points) - minval(x_trans, mask=valid_points)
+            y_range = maxval(y_trans, mask=valid_points) - minval(y_trans, mask=valid_points)
+            plot_scale = max(x_range, y_range)
+        else
+            ! All points are NaN, use default scale
+            plot_scale = 1.0_wp
+        end if
 
         ! Define pattern lengths (matplotlib-like)
         dash_len = plot_scale * 0.03_wp    ! 3% of range
@@ -1502,6 +1530,15 @@ subroutine render_patterned_line(self, plot_idx, linestyle)
         drawing = .true.  ! Start drawing
 
         do i = 1, size(self%plots(plot_idx)%x) - 1
+            ! Skip segment if either point is invalid (NaN)
+            if (.not. valid_points(i) .or. .not. valid_points(i+1)) then
+                ! Reset pattern state when encountering NaN
+                current_distance = 0.0_wp
+                pattern_index = 1
+                drawing = .true.
+                cycle
+            end if
+
             x1_screen = x_trans(i)
             y1_screen = y_trans(i)
             x2_screen = x_trans(i+1)
@@ -1519,7 +1556,7 @@ subroutine render_patterned_line(self, plot_idx, linestyle)
         end do
 
         ! Clean up
-        deallocate(x_trans, y_trans)
+        deallocate(x_trans, y_trans, valid_points)
     end subroutine render_patterned_line
 
     subroutine render_segment_with_pattern(self, x1, y1, x2, y2, segment_length, &