Agreed that I didn't include some of the low points around 2015,...

Agreed that I didn't include some of the low points around 2015, but I also didn't include some of the high prices a few years earlier. Taking the Chart E25 has produced titled 2006 to 2029, 2019 real dollars I looked at what the 2006 to 2015 period average was. It looks to be about US$6.7/dmtu, so if you create a longer-term series you still get US$6ish per dmtu.

Batteries
I agree that the primary demand for Mn is steel, and batteries will add something. The key being that they are adding to Mn demand. That demand boost could be more than expected if battery chemistries shift towards Mn and away from Cobalt/Nickel which is anyone considering the relative cost of both metals would want to do to lower the cost of the battery input materials.

If EV demand increases the price of Nickel, it will make high Nickel grades of stainless steel more expensive. Grades like 201 that have less nickel and include Manganese will have an increased price difference. This will likely increase their use when grade 201 is an acceptable substitute.

Something interesting I found was that the demand for Mn appears to be growing a lot faster than the production of steel and Mn is primarily used in Steel. This means the changes in what steel products are being used by the world is causing Mn demand to grow more quickly, and its not just by a little bit. From E25's charts contained Mn production was 5Mt in 2000 and is about 23MT now. Google tells me world crude steel production was 850.1Mt in 2000 and in 2020 it was 1877MT. Mn has shifted from representing 0.6% of crude steel to 1.2%!!