'The lesser evil' might as well be left (leaning) from the majorities POV. In that case the shift would be to the left. And furthermore you seem to be assuming that this shift continues because you keep voting for the 'lesser evil'?

I think that's contradictory. Voting for someone is telling them you like their course best. Why would they change their course if they are already getting the votes? (Or lead the polls?) They would only do so to capture another parties audience - and only if their own ideas are not popular (enough) already. So the contrary is true: Parties tend towards whoever is getting more votes. This is only logical, because that's ultimately what they need.

Having to vote for a 'lesser evil' just means your system is broken, corrupt, or you feel like you have no other option. In functioning democratic systems, you will see fluctuations based on the general sentiment towards current topics. What's currently going on tends to have a much more significant impact on voters than any ideals.

To give you a very simplistic example: Economy bad -> People vote for guy who (they think) will fix it. This was a big factor in Trumps victory. (And there are probably also more racist then you think.)