Apparently Linux does not have any monitor hotplugging support which is quite a pain. Every time you want to attach a monitor to laptop you have to reconfigure the display layout. This is a tad frustrating if you have to do this several times a day. And it doesn't help that KDE subsystems are a bit flaky when it comes to changing display configuration. I've had plasma crash a on me 1/3 times while performing this operation.

Long story short I got fed up with all of this and wrote the following 3 line script to automate the process and partially alleviate this head ache

 #!/bin/bash
 xrandr --output LVDS1 --auto --output VGA1 --auto
 sleep 1
 kquitapp plasma-desktop &> /dev/null
 sleep 1
 kwin --replace & &> /dev/null
 sleep 1
 kstart plasma-desktop &> /dev/null

You probably need to adjust the xrandr line to make it behave like you want but auto everything works quite well for me. Check man page for xrandr for details.

For further reading on monitor hot plugging I encourage you read launchpad bug #306735. Fortunately there are solutions for this problem, however they are on the other side of the pond.

Update: Added the kwin replace line to fix sporadic malfunction of kwin (disappearance of window decorations) during this whole operation.