Preferences / Schematic editor / Display options / Rendering engine / Accelerated graphics.
Programme came installed with “Fallback graphics”. Needed changing to “Accelerated graphics”.
I found it a bit strange that only the “dot” line version was affected. All the other line versions worked correctly.
Thanks for the comments everyone.
Application: KiCad Schematic Editor
Version: 5.99.0-unknown-4d6b1a4e36~140~ubuntu20.04.1, release build
libcurl/7.68.0 OpenSSL/1.1.1f zlib/1.2.11 brotli/1.0.7 libidn2/2.2.0 libpsl/0.21.0 (+libidn2/2.2.0) libssh/0.9.3/openssl/zlib nghttp2/1.40.0 librtmp/2.3
Platform: Linux 5.4.0-88-generic x86_64, 64 bit, Little endian, wxGTK, cinnamon, x11
Date: Oct 8 2021 18:19:47
wxWidgets: 3.0.4 (wchar_t,wx containers,compatible with 2.8) GTK+ 3.24
Compiler: GCC 9.3.0 with C++ ABI 1013