### file dia2plantuml.tcl
proc dia2plantuml {text {ext svg}} {
    set b64 ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/
    set pml 0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz-_
    set lmapper [list]
    set i 0
    foreach char [split $b64 ""] {
        lappend lmapper $char
        lappend lmapper [string range $pml $i $i]
        incr i
    }
    set b64 [string map $lmapper [binary encode base64 [string range [zlib compress $text] 2 end-4]]]
    set uri https://www.plantuml.com/plantuml/$ext/$b64
    return $uri
}

% source dia2plantuml.tcl
% puts [dia2plantuml "class TclExample"]
https://www.plantuml.com/plantuml/svg/Iyv9B2vM24dESQr8p2t8IGK0