Could not calculate the upgrade

[Prasann]

When I tried to upgrade from ubuntu 14.04 to ubuntu 16.04.
I got the following errors and after this it failed and stopped upgrading.

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

I got these messages earlier also and found that its related with gunplot.
Though I removed all gunplot packages and tried to upgrade I got this error again.
I am not able to upgrade currently. Any help is really appreciated.
Thank you.

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: ubuntu-release-upgrader-core 1:0.220.9
ProcVersionSignature: Ubuntu 3.13.0-108.155-generic 3.13.11-ckt39
Uname: Linux 3.13.0-108-generic x86_64
NonfreeKernelModules: nvidia_uvm nvidia_drm nvidia_modeset nvidia
ApportVersion: 2.14.1-0ubuntu3.23
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: Unity
Date: Sun Feb 5 13:47:59 2017
InstallationDate: Installed on 2013-08-29 (1255 days ago)
InstallationMedia: Ubuntu 12.04.1 LTS "Precise Pangolin" - Release amd64 (20120823.1)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to trusty on 2017-02-05 (0 days ago)