do-release-upgrade -p failed to upgrade my system

[Dilton McGowan II]

Checking package manager
Reading package lists... Done
Building dependency tree
Reading state information... Done
Building data structures... Done

Calculating the changes

Calculating the changes

Could not calculate the upgrade

An unresolvable problem occurred while calculating the upgrade.

This can be caused by:
* Upgrading to a pre-release version of Ubuntu
* Running the current pre-release version of Ubuntu
* Unofficial software packages not provided by Ubuntu

If none of this applies, then please report this bug using the
command 'ubuntu-bug ubuntu-release-upgrader-core' in a terminal.

Restoring original system state

Aborting
Reading package lists... Done
Building dependency tree
Reading state information... Done
Building data structures... Done
=== Command detached from window (Tue Jul 15 14:48:59 2014) ===
=== Command terminated with exit status 1 (Tue Jul 15 14:48:59 2014) ===

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: update-manager 1:0.156.14.15
ProcVersionSignature: Ubuntu 3.13.0-30.55~precise1-generic 3.13.11.2
Uname: Linux 3.13.0-30-generic x86_64
NonfreeKernelModules: nvidia
ApportVersion: 2.0.1-0ubuntu17.6
Architecture: amd64
Date: Tue Jul 15 14:42:02 2014
GsettingsChanges:
 com.ubuntu.update-manager first-run false
 com.ubuntu.update-manager launch-time 1405452796
 com.ubuntu.update-manager show-details true
 com.ubuntu.update-manager window-height 630
 com.ubuntu.update-manager window-width 726
InstallationMedia: Ubuntu 12.04.1 LTS "Precise Pangolin" - Release amd64 (20120823.1)
MarkForUpload: True
PackageArchitecture: all
ProcEnviron:
 LANGUAGE=en_US:en
 TERM=xterm
 PATH=(custom, no user)
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: update-manager
Symptom: ubuntu-release-upgrader-core
UpgradeStatus: Upgraded to precise on 2014-07-15 (0 days ago)

[Dilton McGowan II (diltonm)]

After painstakingly going through my package list that was installed in 12.04 LTS I found that these two packages apparently can't be installed together on 14.04:

gnuplot-nox install
gnuplot-x11 install

Using Synaptic which I trust to give me more details, I find that either can be installed but not both or not both in a single file that I use for Read Markings.

Whether those broke the upgrade from 12.04 LTS to 14.04 LTS, I'm not sure.