After render show or hide axis x not working

``` javascript
var chart = c3.generate({
data: {
columns: [
['data1', 30, 200, 100, 400, 150, 250],
['data2', 50, 20, 10, 40, 15, 25]
],
axes: {
data1: 'y',
data2: 'y2'
}
},
grid: {
x: {
show: true
}
}
});

chart.internal.config.axis_x_show=false;

chart.flush();
```

Start with the c3.generate configuration and the chart.flush call in the reproduction, then trace how axis_x_show is read after rendering. Reproduce the example and verify whether changing that setting updates the x-axis visibility. Done means post-render show and hide changes work consistently, with regression coverage for the demonstrated sequence.

Written by the indexing model from the issue text.