Just thought I could make one like this. You can use pstree to get a text-version in tree style.

Here are the codes:

cat /proc/*/status | awk '
BEGIN {printf("graph G {\n")}
/Name/ {name = $2}
/^Pid/ {pid = $2}
/PPid/ {
  if (PROCINFO["ppid"] != $2) {
    printf("p%d [label = \"%s\"]\n", pid, name);
    if ($2 != 0)
      printf("p%d -- p%d\n", pid, $2);
    }
  }
END {printf("}")}' | circo -T png -o test.png

It outputs all process except the current processes (cat/awk/circo). You can use if ('$$' != $2) { to replace the checking code, $$ is the current Bash shell's process ID, which is awk's parent process ID. Note that part is actually being processed in Bash not awk. The awk code pauses before $$, then continues after.

Next one hide the kernel threads:

cat /proc/*/status | awk '
BEGIN {printf("graph G {\n")}
/Name/ {name = $2}
/^Pid/ {pid = $2}
/PPid/ {
  if ($2 == 2 || pid == 2) {
    printf("p%d [label = \"%s\"]\n", pid, name);
    if ($2 != 0)
      printf("p%d -- p%d\n", pid, $2);
    }
  }
END {printf("}")}' | circo -T png -o test.png

I am not sure if all kernel threads are with PID #2, but it works for me. And if those children had children, then this code will work probably.

The last one only show kernel threads:

cat /proc/*/status | awk '
BEGIN {printf("graph G {\n")}
/Name/ {name = $2}
/^Pid/ {pid = $2}
/PPid/ {
  if ($2 != 2 && pid != 2 && PROCINFO["ppid"] != $2) {
    printf("p%d [label = \"%s\"]\n", pid, name);
    if ($2 != 0)
      printf("p%d -- p%d\n", pid, $2);
    }
  }
END {printf("}")}' | circo -T png -o test.png