Lawrence,
Unfortunately many terminal emulations don't default to the original terminal definitions for graphic characters as they now want to support UTF-8 or other character sets such as Thai or Cyrillic.
In PxPlus we ship with the the original terminal escape sequence or character required to display line graphics. For example if you have a VT100 physical device the standard setting in *dev/vt100 should generate proper line characters -- however this is done using high order ASCII ($80$=>$FF$) and these generally are mapped to something other than the line graphics on most emulators.
For XTERM devices we have custom 'GS' 'GE', and 'GD' mnemonics defined that we validate by running on a Linux console however we know many emulators don't handle them consistent with the Linux definition.
Now you can change the characters/escape sequence sent by the system by defining your own 'GS' (Start graphics), 'GE' (End Graphics), and 'GD' (Graphic characters) mnemonics.
Lastly though, we strongly recommend you switch to using WindX which not only seamlessly handles the graphic codes, it provides must more functionality than is found in most text mode terminal emulators.
Emulating text mode terminals that existed last century while viable, limits the potential capabilities of the clients application.