blt 2.5.3+dfsg-4 (armhf binary) in ubuntu focal

 BLT is a library of useful extensions for the Tcl language and the
 popular Tk graphical toolkit. It adds a vector and tree data type,
 background execution and some debugging tools to Tcl, and provides
 several new widgets for Tk, including graphs, bar-charts, trees, tabs,
 splines and hyper-links, as well as a new geometry manager, drag &
 drop support, and more.
 .
 This package is a dummy package which depends on the current BLT
 library.