During 2013-2018 a lot of by-elections were held due to fake degree cases. Individual by-elections reasons were different but heavy majority was won by PML(N) as they had vote bank plus provincial governments in Punjab & Baluchistan and federal too. Supreme court/tribunal thoughtfully investigated results with months in investigation and called 2013 elections fair as much as possible in Pakistani environment. The difference between two parties vote was in hundreds of thousand in more than 70% constituencies in Punjab.
On a side note: PML(N) rigged elections in 2013 when they were not in power in federal & local bodies government and they were unable to rig elections when in 2018 they recently had all three levels on governments in Punjab i.e Federal, provincial and local bodies.