Using the same notation as in your example, try:
Φm(ϕ, t) = interpolate(range(0, 7, length(ϕ)), ϕ, SteffenMonotonicInterpolation())(t)
Using the same notation as in your example, try:
Φm(ϕ, t) = interpolate(range(0, 7, length(ϕ)), ϕ, SteffenMonotonicInterpolation())(t)