Here is how i do it by using crossfilter and dc.js. You can get the last x days by doing something like:
xrange = d3.extent(data, function(d) { return d.date; });
last60 = [d3.time.day.offset(xrange[1], -60), d3.time.day.offset(xrange[1], 1)]
Assuming you have a date
in your data. Do this after reading the data with d3, but before feeding it into crossfilter.
Then for the bottom chart (mini plot) simply set the filter property to:
.filter(last60)
And also set the xrange for the top chart to the same range:
.x(d3.time.scale().domain(last60))
Something like this might also work for setting the xrange, but i havent had any success with that:
topchart.focus(bottomchart.filter())