How To get rate of taxitem in taxgroup

hi,

I am trying to find rate for individual tax item in tax group.

i have written following code,I have attached screenshot of taxgroup also.

function pageInit()
{
var lineTax = nlapiGetLineItemValue('expense','taxcode',1);
alert('lineTax - '+lineTax);
var taxArr = searchTax(lineTax);

}

function searchTax(taxgrpid)
{
var secArr = new Array();

var filters = new Array();
filters[0] = new nlobjSearchFilter('internalid',null,'is',taxgrpid);

var columns = new Array();
columns[0] = new nlobjSearchColumn('internalid');

var searchResult = new nlapiSearchRecord('taxgroup',null,filters,columns) ;

if(searchResult != null && searchResult != '' && searchResult.length >= 1)
{
alert(searchResult.length); // return correct
var linecount = nlapiGetLineItemCount('taxitem');alert('linecount - '+linecount); //line count returns '-1'. here i am trying to get line item details i.e name,rate,type of individual tax item in tax group.