You can get hold of the shapes once the draw method has been called and set a transform:
...
sidechart.draw();
x.shapes.selectAll("text").attr("transform", "rotate(-45)");
However dimple already uses the transform to move the labels between the ticks and do the rotate, so you might want to append the transform like this:
...
sidechart.draw();
x.shapes.selectAll("text").attr("transform",
function (d) {
return d3.select(this).attr("transform") + " rotate(-45)";
});
I tried this and it offset the labels from where I was expecting, so you may need to add a translate in, but you'll probably need to find an appropriate offset for your own chart, I've used 20 here as an example:
...
sidechart.draw();
x.shapes.selectAll("text").attr("transform",
function (d) {
return d3.select(this).attr("transform") + " translate(0, 20) rotate(-45)";
});