ufucuk.
I had the same problem and I solved it doing the following:
$("#hereGoesMyMinValue").text($("#myGraphId").
children(".jqplotxaxis").children(":first").text());
//first value shown
$("#hereGoesMyMaxValue").text($("#myGraphId").
children(".jqplot-xaxis").children(":last").text());
//last value shown
Note that this will be done only once. Here explains how to sync them
By the way, you also can do the following:
var title=$("#selecting_period").children(".jqplot-title").text();
var coolTitle=
title+": Showing "+ $("#myGraphId").children(".jqplot-xaxis").
children(":first").text()+" to "+$("#myGraphId").
children(".jqplot-xaxis").children(":last").text();
$("#myGraphId").children(".jqplot-title").text(coolTitle);