D3 line smoothing (stronger than monotonous)

var valueline = d3.svg.line()
.interpolate("monotone") 
.x(function(d) { return x(d.t); })
.y(function(d) { return y(d.r); });

This will cause the line to become smoother.

Is there a way to control the smoothing power?

Other string values ​​of interpolation functions (for example: basis, cardinal ...) give good smoothing, but also change the shape of the line.

I need something like "monotonous", but stronger. Make corners even and rounded.

the documentation shows that I can use a custom interpolation function, but mentions very little about how to do this.