Can't use comma as separator c3js

Question

I'm currently working on a evolution graph with the framework c3.js based on d3.js.

I want to format the values of the Y axis that are numbers. I currently have this format : XX.X What I want is to use the comma as separator of decimals : XX,X

I tried two methods :

y : {
            tick : {
                values : steps, 
                format : d3.format(",.1f")
            },
            min : steps[0],
            max : steps[steps.length-1]
         }

And to use the d3 formatter :

 y : {
                tick : {
                    values : steps, 
                    format : function(d) {
                        var s = d3.formatSpecifier("f");
                        s.precision = precisionFixed(0.1);
                        s.comma = true;
                        var f =  d3.format(s);
                        return f(d);
                     }
                },
                min : steps[0],
                max : steps[steps.length-1]
             }

See doc here: https://github.com/d3/d3-format

But I get an error (d3.formatSpecifier is not a function...). Does someone already resolved this issue?

Thank you