There's no built-in support for this (you should star issue 368). But you can simulate it by defining an axisValueFormatter which returns the empty string for non-integers:
axes: {
x: {
axisLabelFormatter: function(num, gran, opts, g) {
if (num == Math.floor(num)) {
return Dygraph.numberAxisLabelFormatter(num, gran, opts, g);
} else {
return '';
}
}
}
}
Fully-functioning demo here.