Update

indent/plantuml.vim

@@ -0,0 +1,58 @@
+if !exists('g:polyglot_disabled') || index(g:polyglot_disabled, 'plantuml') == -1
+  
+if exists("b:did_indent")
+  finish
+endif
+let b:did_indent = 1
+
+setlocal indentexpr=GetPlantUMLIndent()
+setlocal indentkeys=o,O,<CR>,<:>,!^F,0end,0else,}
+
+" only define the indent code once
+if exists("*GetPlantUMLIndent")
+  finish
+endif
+
+let s:incIndent =
+            \ '^\s*\(loop\|alt\|opt\|group\|critical\|else\|legend\|box\)\>\|' .
+            \ '^\s*\([hr]\?note\|ref\)\>[^:]*$\|' .
+            \ '^\s*title\s*$\|' .
+            \ '^\s*skinparam\>.*{\s*$\|' .
+            \ '^\s*state\>.*{'
+
+let s:decIndent = '^\s*\(end\|else\|}\)'
+
+function! GetPlantUMLIndent(...) abort
+    "for current line, use arg if given or v:lnum otherwise
+    let clnum = a:0 ? a:1 : v:lnum
+
+    if !s:insidePlantUMLTags(clnum)
+        return indent(clnum)
+    endif
+
+    let pnum = prevnonblank(clnum-1)
+    let pindent = indent(pnum) 
+    let pline = getline(pnum)
+    let cline = getline(clnum)
+
+    if cline =~ s:decIndent
+        if pline =~ s:incIndent
+            return pindent
+        else
+            return pindent - shiftwidth()
+        endif
+
+    elseif pline =~ s:incIndent
+        return pindent + shiftwidth()
+    endif
+
+    return pindent
+
+endfunction
+
+function! s:insidePlantUMLTags(lnum) abort
+    call cursor(a:lnum, 1)
+    return search('@startuml', 'Wbn') && search('@enduml', 'Wn')
+endfunction
+
+endif