You are looking for the "tickFormatter" option in the API .
For instance:
var data1 = [[0,3],[10,1],[20,2],[40,8],[50,10]];
someFunc = function(val, axis){
return "$" + val
}
plot = $.plot($("#placeholder"),
[{ data: data1}], {
xaxis: { tickFormatter: someFunc }
});
It produces:

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