diff --git a/acinclude.m4 b/acinclude.m4
index 75fcb0d..731fb07 100644
--- a/acinclude.m4
+++ b/acinclude.m4
@@ -51,7 +51,7 @@ AC_DEFUN([PSPP_LIBPLOT],
     # Check whether we can link against libplot if we also link X.
     if test x"$LIBPLOT_LIBS" = x""; then
       AC_PATH_XTRA
-      extra_libs="-lXaw -lXmu -lXt $X_PRE_LIBS -lXext -lX11 $X_EXTRA_LIBS -lm"
+      extra_libs="-lpng -lz -lXaw -lXmu -lXt $X_PRE_LIBS -lXext -lX11 $X_EXTRA_LIBS -lm"
       AC_CHECK_LIB(plot, pl_newpl_r,
       		   [LIBPLOT_LIBS="-lplot $extra_libs"
                     LDFLAGS="$LDFLAGS $X_LIBS"],,