Describe the % of Votes won by the 2 Main Parties since 1979 GE

-Overall from 1979 [81%] to 2010 [65%] it had Steadily Declined with only a Hiccup in 1992 with a 3% Increase -But then 2015 saw a 2% Increase [67%] and then 2017 saw an Enormous 82%! -2019 only managed to get 76%, which is the 3rd Highest Vote Share since 1979 [Higher than 1983]