They get more liberal overtime as well. The court is partisan. Even the current. You can look at 5-4 and majority decisions and there are tons of flip flopping by different justices.
That's correct. The court is partisan and the general trend for each justice is toward the more liberal. Note, however, that in the diagrams contained in the linked wiki article, no justice who starts above 0 (the "conservative" area) ever dips below 0 into liberal territory. Conservatives stay conservative and liberals get more liberal.
Having an ideological preference is not the same thing as being partisan, nor is it evidence thereof. These are different things.
Someone can be partisan and not have an ideological preference. (eg. party employees, pandering politicians)
Someone can have an ideological preference and not be partisan. (eg. religious leaders, philosophers)
If you want to demonstrate that SCOTUS members are partisan, you need demonstrate they voted for the a party's preferences instead of their own ideological preference. If you are instead demonstrating that they voted in line with their own ideological preference, you are simply pointing out that humans have belief systems.