Use the `png_jmpbuf' accessor, as recommended since libpng 1.4.0:
http://www.libpng.org/pub/png/src/libpng-1.2.x-to-1.4.x-summary.txt .

--- plotutils-2.6/libplot/z_write.c	2013-07-12 17:19:12.000000000 +0200
+++ plotutils-2.6/libplot/z_write.c	2013-07-12 17:19:07.000000000 +0200
@@ -164,7 +164,7 @@ _pl_z_maybe_output_image (S___(Plotter *
     }
 
   /* cleanup after libpng errors (error handler does a longjmp) */
-  if (setjmp (png_ptr->jmpbuf))
+  if (setjmp (png_jmpbuf (png_ptr)))
     {
       png_destroy_write_struct (&png_ptr, (png_info **)NULL);
       return -1;
@@ -444,7 +444,7 @@ _our_error_fn_stdio (png_struct *png_ptr
 #endif
     }
 
-  longjmp (png_ptr->jmpbuf, 1);
+  longjmp (png_jmpbuf (png_ptr), 1);
 }
 
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<tr><td><span title='2021-12-13 17:48:25 +0100'>2021-12-13</span></td><td><a href='/guix/commit/gnu/packages/ed.scm?id=8394619baceb118df92e355377fd543bb1aa501a'>gnu: Simplify package inputs.</a><span class='msg-avail'>...<span class='msg-tooltip'>This commit was obtained by running:

  ./pre-inst-env guix style

without any additional argument.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2021-01-15 14:06:12 +0100'>2021-01-15</span></td><td><a href='/guix/commit/gnu/packages/ed.scm?id=dcaa934c3db2c8a2ad7819566585ef4a8308ef23'>gnu: ed: Update to 1.17.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/ed.scm (ed): Update to 1.17.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-03-05 23:36:06 +0100'>2020-03-05</span></td><td><a href='/guix/commit/gnu/packages/ed.scm?id=4a50e983bade29792076975092fec60df5cca59d'>gnu: ed: Update to 1.16.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/ed.scm (ed): Update to 1.16.
</span></span></td><td>Marius Bakke</td></tr>