Is the Health App Graph Logarithmic?
It doesn't look logarithmic to me. 

Rounding the Steps figures, 20k peak, average 10k - the 10k line is half way up the scale, give or take.
If it were logarithmic it would be significantly off-centre. 

I think it shows some non-colour-differentiated headroom above the high-point, which might be giving the impression it's not linear.

On my own phone, my high-point is about 3x my daily average. Allowing for that 'header' space above the high-point, my average line is showing as about ⅓ of the way up the scale; confirming my impression that it is linear, plus a header.