How to change tooltip label according to number in c3 chart?

I want to display tooltip label as a text in place of number, I have fix six string like A = 1, B = 2, C = 3, D = 4, E = 5, E = 6.

I am using number because i can't plot the graph with text string so i am using number from 1 to 6. Please help me. I am using c3 chart in laravel5.

Here is the code 

c3.generate({
        bindto: '#activity-user-grade-chart',
        size: {
            height: 240,
            width: 480
        },
        data: {
            x: 'x',
            xFormat: '%d-%m', // how the date is parsed
            type: 'bar',
            columns: [
                ['01-08','01-09','01-10','01-11'],
                [1,4,3,5]
            ]
        },
        color: {
            pattern: ['#FEBE29']
        },
        axis: {
            x: {
                type: 'timeseries',
                tick: {
                    format: '%d-%m' // how the date is displayed
                }
            }

        }
    }); 

via Chebli Mohamed